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Phone: (671) 646-3131 Fax: (671) 649-6178
Department of Public Works 542 North Marine Corp
Tamuning
Guam 96913
AUGUST 2010
GUAM TRANSPORTATION STORMWATER DRAINAGE MANUAL
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ACKNOWLEDGEMENTS
This Transportation Stormwater Drainage Manual discusses approaches, methods, and assumptions applied
in the design and analysis of highway drainage structures. PARSONS TRANSPORTATION GROUP developed
this manual for the Guam Department of Public Works (DPW). This manual is intended for use by DPW’s
highway staff involved in design and construction projects, and consultants and contractors involved in
projects that require work within DPW highway rights-of-way, or projects that connect or discharge to DPW
highways.
We wish to acknowledge the Guam Environmental Protection Agency and DPW for their significant
contributions in the production of this manual. The development of this manual was facilitated and funded by
the Federal Highway Administration (FHWA).
Prepared By-
PARSONS TRANSPORTATION GROUP
Parsons
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TOC - 1 August 2010
TABLE OF CONTENTS
TABLES AND FIGURES
ACRONYMS AND ABBREVIATIONS
GLOSSARY
1 PURPOSE AND SCOPE ........................................................................................................ 1-1 1.1 THE IMPORTANCE OF STORMWATER MANAGEMENT 1.2 MANAGEMENT OF RUNOFF FROM TRANSPORTATION PROJECTS 1.3 ORGANIZATION OF THIS MANUAL 1.4 HOW TO USE THIS MANUAL 1.5 REVIEW PROCESS AND REGULATORY STANDING OF THIS MANUAL
2 STORMWATER PLANNING AND GUAM DRAINAGE POLICIES ........................................ 2-1 2.1 DRAINAGE MASTER PLAN AND PROJECT DEVELOPMENT 2.2 SPECIAL DESIGN CONSIDERATIONS 2.3 OVERVIEW OF FEDERAL, TERRITORIAL, AND LOCAL ORDINANCES 2.4 DRAINAGE SUBMITTALS AND REVIEW PROCEDURES 2.5 SUBMITTAL REQUIREMENTS FOR DRAINAGE REVIEW
3 MINIMUM REQUIREMENTS .................................................................................................. 3-1 3.1 INTRODUCTION 3.2 MINIMUM REQUIREMENTS
4 HYDROLOGICAL ANALYSIS ................................................................................................ 4-1 4.1 GENERAL HYDROLOGY 4.2 DESIGN STORM SELECTION 4.3 TIME OF CONCENTRATION 4.4 PROCEDURES FOR DETERMINING PEAK RUNOFF FLOW
5 HYDRAULICS ANALYSIS ..................................................................................................... 5-1 5.1 GENERAL 5.2 FREQUENCY POLICY 5.3 ROADWAY DRAINAGE 5.4 OPEN CHANNELS AND DITCHES 5.5 STORM DRAINS 5.6 CROSS DRAINAGE
6 PERMANENT BEST MANAGEMENT PRACTICES .............................................................. 6-1 6.1 INTRODUCTION 6.2 TYPES AND FUNCTIONS OF BMPs 6.3 DESIGN POLLUTION PREVENTION BMPs 6.4 QUALITY TREATMENT BMPs 6.5 FLOW CONTROL BMPs 6.6 STORMWATER FACILITY COMPONENTS
TOC - 2 August 2010
6.7 OPERATION AND MAINTENANCE
7 CONSTRUCTION SITE BEST MANAGEMENT PRACTICES ............................................... 7-1 7.1 SOIL STABILIZATION PRACTICES 7.2 TEMPORARY SEDIMENT CONTROL PRACTICES 7.3 WIND EROSION CONTROL PRACTICES 7.4 TRACKING CONTROL BMPs 7.5 NON-STORMWATER MANAGEMENT PRACTICES
BIBLIOGRAPHY
APPENDIX I .................................................................................................................................. I-1
APPENDIX IA ............................................................................................................................... IA-1
APPENDIX II ................................................................................................................................. II-1
APPENDIX III ................................................................................................................................ III-1
TABLES AND FIGURES - 1 August 2010
TABLES AND FIGURES 1
2
LIST OF TABLES 3
CHAPTER 1 4
Table 1-1 BMP Classifications 5
CHAPTER 2 6
Table 2-1 Project Stormwater Development Process 7
CHAPTER 3 8
CHAPTER 4 9
Table 4-1 Guidelines for Selecting Peak Runoff Rate and Flood Hydrograph 10 Methods 11
Table 4-2 One-Hour Storm Distribution 12
Table 4-3 Rainfall Intensity Equations for a One-Hour Duration Storm 13
Table 4-4 SBUH Spreadsheet Method 14
Table 4-5 USGS Regression Equations for Guam 15
Table 4-6 Frequency of Flows for Nine Rivers on Guam 16
CHAPTER 5 17
Table 5-1 Recommended Frequency for Design of Hydraulic Structures 18
Table 5-2 Recommended Pavement Cross Slope 19
Table 5-3 Allowable Spread Criteria 20
Table 5-4 Inlet Spacing Computation Sheet 21
Table 5-5 Permissible Velocities for Various Linings 22
Table 5-6 Storm Drain Calculation Sheet 23
Table 5-7a Fill Height, Concrete Pipe 24
Table 5-7b Concrete Pipe for Shallow Cover Installations 25
Table 5-7c Thermoplastic Pipe, Fill Height 26
CHAPTER 6 27
Table 6-1 Permanent BMP Categories 28
Table 6-2 Permanent BMP Numbering Conventions 29
TABLES AND FIGURES - 2 August 2010
Table 6-3 Manning’s n for BMP RT.03 Vegetated Filter Strips 1
Table 6-4 Manning’s Coefficient in Basic, Wet, and Continuous Biofiltration Swales 2
Table 6-5 Biofiltration Swale Sizing Criteria 3
Table 6-6 Media Filter Drain Mix 4
Table 6-7 Media Filter Drain Widths 5
Table 6-8 Distribution of Depths in Wetland Cell 6
Table 6-9 Rock Protection at Outfalls 7
CHAPTER 7 8
Table 7-1 Construction Site BMPs by Construction Activity 9
Table 7-2 Temporary Soil Stabilization BMPs 10
Table 7-3 Application Rates for Guar Stabilizer 11
Table 7-4 Application Rates for Copolymers 12
Table 7-5 Properties of Soil Binders for Erosion Control 13
Table 7-6 Temporary Sediment Control BMPs 14
Table 7-7 Tracking Control BMPs 15
Table 7-8 Non-Stormwater BMPs 16
Table 7-9 Maintenance BMPs 17
18
LIST OF FIGURES 19
CHAPTER 1 20
CHAPTER 2 21
CHAPTER 3 22
Figure 3-1 Water Quality Classification - Water Bodies of Guam 23
CHAPTER 4 24
Figure 4-1 Intensity Duration Frequency (IDF) Curves for a One-Hour Duration 25 Storm 26
Figure 4-2 Design Rainfall Distribution for a One-Hour Duration Storm 27
Figure 4-3 Rainfall (inches) per Recurrence Period for a One-Hour Duration Storm 28
29
TABLES AND FIGURES - 3 August 2010
CHAPTER 5 1
Figure 5-1 Typical Gutter Sections 2
Figure 5-2 Perspective View of Gutter and Curb Opening Inlets 3
Figure 5-3 Flanking Inlets 4
Figure 5-4 Commonly-Used Culvert Shapes 5
Figure 5-5 Projecting End 6
Figure 5-6 Beveled End Section 7
Figure 5-7 Flared End Section 8
Figure 5-8 Slope Collar 9
Figure 5-9 Headwall with Wingwalls for Circular Pipe 10
Figure 5-10 Splash Pad Details 11
CHAPTER 6 12
Figure 6-1 Flow Chart – Selection of Design Pollution Prevention BMP 13
Figure 6-2 Flow Chart – Selection of Flow Control BMP 14
Figure 6-3 Flow Chart – Selection of Runoff Treatment BMP 15
Figure 6-4 Biofiltration Pond 16
Figure 6-5 Infiltration Pond 17
Figure 6-6 Parking Lot Perimeter Trench Design 18
Figure 6-7 Infiltration Trench System 19
Figure 6-8 Median Strip Trench Design 20
Figure 6-9 Oversize Pipe Trench Design 21
Figure 6-10 Underground Trench and Grit Chamber 22
Figure 6-11 Observation Wells 23
Figure 6-12 BMP IN.04 - Infiltration Vault 24
Figure 6-13 BMP IN.05 - Drywell 25
Figure 6-14 BMP RT.01 - Natural Dispersion 26
Figure 6-15 BMP RT.02 - Engineered Dispersion Area 27
Figure 6-16 Narrow-Area Vegetated Filter Strip Design 28
Figure 6-17 BMP RT.04 - Biofiltration Swale Plan 29
TABLES AND FIGURES - 4 August 2010
Figure 6-18 BMP RT.04 - Biofiltration Swale Section 1
Figure 6-19 Biofiltration Swale Underdrain Detail 2
Figure 6-20 Biofiltration Swale Details 3
Figure 6-21 BMP RT.05 – Wet Biofiltration Swale 4
Figure 6-22 BMP RT.06 - Continuous Inflow Biofiltration Swale 5
Figure 6-23 BMP RT.07 - Media Filter Drain Type 1 6
Figure 6-24 BMP RT.07 - Media Filter Drain Type 2 7
Figure 6-25 BMP RT.07 - Media Filter Drain Type 3 8
Figure 6-26 BMP RT.07 - Media Filter Drain Type 4 9
Figure 6-27 BMP RT.08 - Wet Pond 10
Figure 6-28 Wet Pond Cross Sections 11
Figure 6-29 BMP RT.10 - Combined Detention and Wet Pond – Plan View 12
Figure 6-30 Combined Detention and Wet Pond – Cross Sections 13
Figure 6-31 Alternative Configuration of Detention and Wet Pond 14
Figure 6-32 BMP RT.09 Constructed Wetland, Option A 15
Figure 6-33 BMP RT.09 Constructed Wetland, Option B 16
Figure 6-34 BMP FC.01 Detention Pond 17
Figure 6-35 Detention Pond – Cross Sections 18
Figure 6-36 Outlet Control Structure, Option A 19
Figure 6-37 Outlet Control Structure, Option B 20
Figure 6-38 Alternate Overflow Structure and Trash Rack (Birdcage) 21
Figure 6-39 Typical Pre-settling/Sedimentation Basin 22
Figure 6-40 Pre-settling/Sedimentation Basin – Alternate Sections 23
Figure 6-41 Flow Splitter 24
Figure 6-42 Flow Spreader – Anchor Plate 25
Figure 6-43 Flow Spreader – Notched Curb Spreader 26
Figure 6-44 Flow Spreader – Through-Curb Port 27
Figure 6-45 Flow Spreader Trench Type 1 28
Figure 6-46 Flow Spreader Trench Type 2 29
TABLES AND FIGURES - 5 August 2010
Figure 6-47 Pipe/Culvert Outlet Rock Protection 1
Figure 6-48 Gabion Outlet Protection 2
CHAPTER 7 3
Figure 7-1 Weir Tank 4
Figure 7-2 De-Watering Tanks 5
Figure 7-3 Typical Ford Crossing 6
Figure 7-4 In-Stream Erosion and Sediment Control Isolation Techniques 7
Figure 7-5 Gravel Berm with Impermeable Membrane 8
Figure 7-6 Gravel Bag Technique 9
Figure 7-7 In-Stream Erosion and Sediment Control Isolation 10
Figure 7-8 K-Rail 11
12
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ACRONYMS - 1 August 2010
ACRONYMS AND ABBREVIATIONS
Acronym/Abbreviation Term
AASHTO American Association of State Highway and Transportation Officials
ACP Asphalt Concrete Pavement
AHD Allowable Headwater Depth
AHW Allowable High Water
ASTM American Society for Testing and Materials
BFM Bonded Fiber Matrix
BMPs Best Management Practices
BOD Biochemical Oxygen Demand
BST Bituminous Surface Treatment
CAVFS Compost-Amended Vegetated Filter Strip
CCS Cellular Confinement System
CEC Cation Exchange Capacity
CERCLA Comprehensive Environmental Response, Compensation, and Liability Act
CESCL Certified Erosion and Sediment Control Lead
CN Curve Number
COE U.S. Army Corps of Engineers
CSBC Crushed Surfacing Base Course
CWA Clean Water Act
DAWR Guam Division of Aquatic and Wildlife Resources
DHW Design High Water
DPW Department of Public Works
DSA Disturbed Soil Area
EGL Energy Grade Line
EPA U.S. Environmental Protection Agency
EPP Environmental Protection Plan
ESA Endangered Species Act
ESC Erosion and Sediment Control
FAA Federal Aviation Administration
FC Freeboard Check
FEMA Federal Emergency Management Agency
ACRONYMS - 2 August 2010
FHWA Federal Highway Administration
GEPA Guam Environmental Protection Agency
HDPE High-Density Polythethylene
HGL Hydraulic Grade Line
IDF Intensity-Duration-Frequency curve
IPM Integrated Pest Management
MEP Maximum Extent Practicable
MFD Media Filter Drain
N Newton’s Measure of Force
NCHRP National Cooperative Highway Research Program
NEPA National Environmental Policy Act
NOAA National Oceanic and Atmospheric Administration
NPDES National Pollutant Discharge Elimination System
NPS Non-Point Source Pollution
NRCS USDA National Resources Conservation Service (Formerly known as the Soil Conservation Service or SCS)
NWI National Wetlands Inventory
OHW or OHWL Ordinary High Water Level
PCC Portland Cement Concrete
PCCP Portland Cement Concrete Pavement
PGIS Pollution-Generating Impervious Surfaces
PLS Pure Live Seed
PS&E Plans, Specifications, and Estimates
RCRA Resource Conservation Recovery Act
RE Resident Engineer
RECP Rolled Erosion Control Product
RSP Rock Slope Protection
SA Surface Area
SBUH Santa Barbara Urban Hydrograph method
SCA Sanitary Control Area
SCR Special Contract Requirements
SDM Stormwater Drainage Manual
SDWA Safe Drinking Water Act
SF Safety Factor
SPCC Spill Prevention, Control, and Countermeasures
ACRONYMS - 3 August 2010
SWPE Solid Wall Polyethylene
SWPPP Stormwater Pollution Prevention Plan
Tc Time of Concentration
TESC Temporary Erosion and Sediment Control
TLUC Territorial Land Use Commission
TSS Total Suspended Solids
UH Unit Hydrograph
UIC Underground Injection Control
USACE U.S. Army Corps of Engineers
USDA U.S. Department of Agriculture
USDOT U.S. Department of Transportation
USFWS U.S. Fish and Wildlife Service
USGS U.S. Geological Survey
UV Ultraviolet
WHPP Wellhead Protection Program
WPC Water Planning Committee
WQS Water Quality Standards
WRCA Guam Water Resources Conservation Act
WSEL Water Surface Elevations
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GLOSSARY - 1 August 2010
GLOSSARY
Term Definition
80 percent capture rule
The 80 percent capture rule is based on an analysis of the rainfall frequency spectrum. It is the impervious surface runoff from a design storm that produces 80 percent of all average annual rainfall from a statistical viewpoint.
90 percent capture rule
The 90 percent capture rule is based on an analysis of the rainfall frequency spectrum. It is the impervious surface runoff from a design storm that produces 90 percent of all average annual rainfall from a statistical viewpoint.
Areal Extent The magnitude of an area.
Artificial Channels Man-made channels, including ditch, interceptor ditch, swale, median ditch, outfall ditch, lateral ditch, and canal.
Backwater A body of water in which the flow is slowed or turned back by an obstruction such as a bridge or dam, an opposing current, or the movement of the tide.
Baffle A vertical divider placed across the entire width of the wet pond, stopping short of the bottom.
Biofiltration
A pollution control technique using living material to capture and biologically degrade process pollutants. Common uses include processing wastewater, capturing harmful chemicals or silt from surface runoff, and microbiotic oxidation of contaminants in the air.
Biofiltration Swales Vegetation-lined channels designed to remove suspended solids from stormwater.
Bioinfiltration A surface infiltration system covered with grass, but possibly trees or shrubs.
Bioinfiltration Ponds Also known as bioinfiltration swales, combine filtration, soil sorption, and uptake by vegetative root zones, removes stormwater pollutants by percolation into the ground.
Buffer Strip Refers to an area of natural indigenous vegetation that can be enhanced or preserved as part of a riparian buffer or stormwater dispersion system.
Check Dams
Reduce scour and channel erosion by reducing flow velocity and encouraging sediment settlement. A check dam is a small device constructed of rock, gravel bags, sandbags, fiber rolls, or other proprietary product placed across a natural or man-made channel or drainage ditch.
Clear Water Diversion
A system of structures and measures that intercept clear surface water runoff upstream of a project site, transport it around the work area, and discharge it downstream, with minimal water quality degradation for either the project construction operations or construction of the diversion.
GLOSSARY - 2 August 2010
Combination Inlets Consist of both a curb opening inlet and a grate inlet placed in a side-by-side configuration, but the curb opening may be located in part upstream of the grate.
Critical Areas Defined as wetlands, floodplains, aquifer recharge areas, geologically hazardous areas, and those areas necessary for fish and wildlife conservation.
Crown A crown is defined as the highest point of the internal surface of the transverse cross section of a pipe.
Curb Opening Inlets Vertical openings in the curb covered by a top slab.
Development
The term development encompasses all categories of construction and earth-moving, as well as other types of land use and water-oriented construction. For the purposes of roadways, development is defined as the creation of new impervious area.
Downgradient The direction that stormwater flows.
Engineered Dispersion
Similar to natural dispersion, this BMP can be used for impervious or pervious surfaces that are graded to drain via sheet flow or are graded to collect and convey stormwater to engineered dispersion areas after going through a flow-spreading or energy-dissipating device. Engineered dispersion uses the existing vegetation or landscaped areas, existing soils or engineered compost-amended soils, and topography to effectively provide flow control and runoff treatment. Site selection is very important to the success of this BMP.
Eutrophication
The enrichment of water by nutrients, especially nitrogen and/or phosphorous, which can cause accelerated growth of algae and higher plant life that produce changes in the ecological balance and deterioration in water quality.
Evapotranspiration
The total loss of water from a crop into the air. Water evaporates from any moist surface into the air unless the air is saturated. Water surfaces in contact with air, such as lakes, plant leaves, and moist soils, all evaporate water.
Fiber Rolls
Consists of wood excelsior, rice or wheat straw, or coconut fibers rolled or bound into a tight tubular roll and placed on the toe and face of slopes to intercept runoff, reduce its flow velocity, release the runoff as sheet flow, and provide removal of sediment from the runoff.
Flow Control BMPs
Reduce the peak rate of runoff during a storm event by storing the flow and releasing it at a slower rate, thus protecting stream ecosystems from excessive erosion (typical examples are detention ponds and dry vaults).
Flow Splitter
A hydraulic structure used to maintain the stormwater quality treatment volume or flow to an off-line treatment facility while bypassing larger flows. Usually a manhole or catch basin structure having internal weir and orifice controls.
Frontal Flow The water flowing in a section of gutter inlet occupied by a grate.
GLOSSARY - 3 August 2010
Froude Number (Fr)
A dimensionless number comparing inertia and gravitational forces. It may be used to quantify the resistance of an object moving through water, and compare objects of different sizes. Named after William Froude, the Froude number is based on his speed/length ratio.
Grate Inlets Consist of an opening in the gutter or ditch, covered by a grate.
Gravity Bag Filter Is also referred to as a de-watering bag and is a square or rectangular bag made of non-woven geotextile fabric that collects sand, silt, and fines.
Guar Guar is a nontoxic, biodegradable, natural galactomannan-based hydrocolloid treated with dispersant agents for easy field mixing.
Hydraulic Matrix A combination of wood fiber mulch and a tackifier applied as a slurry.
Hydraulic Mulch
Hydraulic mulch consists of applying a mixture of shredded wood fiber or a hydraulic matrix and a stabilizing emulsion or tackifier with hydroseeding equipment, which temporarily protects exposed soil from erosion by raindrop impact or wind. This is one of five temporary soil stabilization alternatives to consider.
Hydrology
For the Highway Designer, the primary focus of hydrology is the water that moves on the Earth’s surface and, in particular, that part that crosses transportation arterials (i.e., highway stream crossings). Hydrology is the science and art of converting rainfall to runoff for design purposes.
Hydroperiod The cyclical changes in the amount or stage of water in an aqueous habitat.
Hydroseeding
Typically consists of applying a mixture of wood fiber, seed, fertilizer, and stabilizing emulsion with hydromulch equipment, which temporarily protects exposed soils from erosion by water and wind. This is one of five temporary soil stabilization alternatives to consider.
Hyetograph A graphical representation of the distribution of rainfall over time.
Infiltration The preferred method for flow control and runoff treatment and offers the highest level of pollutant removal.
Infiltration Ponds
Infiltration ponds are usually earthen impoundments used for the collection, temporary storage, and infiltration of incoming stormwater runoff to groundwater. Infiltration ponds can also be designed to provide runoff treatment.
Isolation Techniques Methods that isolate near-shore work from a water body.
Kinematic Wave Equations A mathematical equation used to determine overland flow travel times.
GLOSSARY - 4 August 2010
Manning’s Equation
The Manning equation is an open-channel flow equation used to find either the depth of flow or the velocity in the channel where the channel roughness, slope, depth, and shape remain constant (steady uniform flow).
Mean High-Water Mark The highest point that the water may reach for a navigable waterway, lake, or ocean.
Mean Low-Water Mark The average low tide.
Media Filter Drain (MFD)
A linear flow-through stormwater runoff treatment device that can be sited along highway side slopes. MFDs have four basic components: a gravel no-vegetation zone, a grass strip, the MFD mix bed, and a conveyance system for flows leaving the MFD mix.
Minimum Requirements Nine requirements intended to achieve compliance with federal and Guam water quality regulations, while catering to the unique linear features of most transportation projects.
National Pollutant Discharge Elimination System (NPDES) Construction usually requires a NPDES permit.
National Wetlands Inventory (NWI)
Provides information on the characteristics, extent, and status of the nation's wetlands and deep water habitats and other wildlife habitats. The inventory is conducted by the U.S. Fish and Wildlife Service.
Nomograph A graph consisting of three coplanar curves, each graduated for a different variable so that a straight line cutting all three curves intersects the related values of each variable.
Oil Containment Boom A weather-resistant, hydrophobic, absorbent-filled boom for removing hydrocarbon sheens from water.
Open Channel
An open channel is a watercourse that allows part of the flow to be exposed to the atmosphere. Classified as those which occur naturally or those which are man-made or improved natural channels.
Operational BMPs Nonstructural practices that prevent or reduce pollutants from entering stormwater.
Ordinary High-Water or Ordinary High-Water Level (OHWL)
For stormwater design purposes, this level is identified by environmental biologists based on vegetation types. The surface boundary is then identified, surveyed, and used as the control for project impacts to the waterbody.
Overside Drains Overside drains are conveyance systems used to protect slopes against erosion.
Permanent BMPs Permanent BMPs are used to quality treat or flow control highway stormwater for the design life of the project site.
Piezometer A small-diameter observation well used to measure the hydraulic head of groundwater in aquifers.
Plans, Specifications, and Estimates (PS&E) Plans, Specifications, and Estimates usually relating to a final set of construction contract documents.
GLOSSARY - 5 August 2010
Pollution-Generating Impervious Surfaces (PGIS)
PGIS is any impervious surface expected to generate pollutants, such as bike lanes, travel lanes, and shoulders. An example of an impervious surface not expected to generate pollutants is sidewalks that are sloped away from the roadway. If the sidewalk slopes towards the roadway, then it contributes to the quantity of stormwater coming from the PGIS.
Presettling Basin Process of allowing stormwater to settle the larger pollution particulates prior to entering a primary stormwater quality treatment or flow control facility.
Presumptive Approach
Meaning that projects that follow the stormwater BMPs contained in the SDM are presumed to have satisfied the water quality requirements and are not required to provide technical justification to support the selection of BMPs.
Projecting End A projecting end is a treatment where the culvert is simply allowed to protrude out of the embankment.
Psyllium
Psyllium is composed of the finely ground mucilloid coating of plantago seeds that is applied as a dry powder or in a wet slurry to the surface of the soil.
Regression Equations The equation representing the relation between selected values of one variable and observed values of the other.
Reynold’s Number
A dimensionless number, the ratio of intertial forces to viscous forces, equal to pvl/n where p is density, v is velocity, n is viscosity relative to some length l, e.g., radius of pipe. For steady flow, the flow lines take the same form at a given Reynold’s number.
Sandbag Barrier
A temporary linear sediment barrier consisting of stacked sandbags, designed to intercept and slow the flow of sediment-laden sheet flow runoff.
Santa Barbara Urban Hydrograph method (SBUH)
The SBUH method is based on the curve number (CN) approach, and also uses USDA National Resources Conservation Service equations for computing soil absorption and precipitation excess. The SBUH method converts the incremental runoff depths into instantaneous hydrographs which are then routed through an imaginary reservoir with a time delay equal to the basin time of concentration.
Sediment/Desilting Basin
A sediment/desilting basin is a temporary basin formed by excavating and/or constructing an embankment so that sediment-laden runoff is temporarily detained under quiescent conditions, allowing sediment to settle out before the runoff is discharged.
Sediment Trap
A temporary containment area that allows sediment in collected stormwater to settle out during infiltration or before the runoff is discharged through a stabilized spillway.
GLOSSARY - 6 August 2010
Silviculture
The art and science of controlling the establishment, growth, composition, health, and quality of forests to meet diverse needs and values of the many landowners, societies and cultures.
Slotted Inlets Consist of a pipe cut along the longitudinal axis, with bars perpendicular to the opening to maintain the slotted opening.
Slope Drain
A pipe used to intercept and direct surface runoff or groundwater into a stabilized watercourse, trapping device, or stabilized area.
Slope Rounding A design technique to minimize the formation of concentrated flows.
Soil Binders
Materials applied to the soil surface to temporarily prevent water-induced erosion of exposed soils on construction sites. Soil binders also provide temporary dust, wind, and soil stabilization (erosion control) benefits.
Source Control BMPs Include operational and structural BMPs. See also operational BMPs and structural BMPs.
Spill Prevention, Control, and Countermeasures (SPCC) A set of plans that must be prepared and followed by the contractor, once approved.
Starch Starch is non-ionic, cold-water soluble (pre-gelatinized) granular cornstarch.
Storm Drain
The portion of the highway drainage system that receives surface water through inlets and conveys the water through conduits to an outfall. It is composed of different length or sizes of pipe or conduit connected by appurtenant structures.
Stormwater Pollution Prevention Plan (SWPPP) The SWPPP is prepared by the contractor and includes the TESC and SPCC plans.
Straw Bale Barrier
A temporary linear sediment barrier consisting of straw bales, designed to intercept and slow sediment-laden sheet flow runoff.
Straw Mulch
Straw mulch consists of placing a uniform layer of straw and incorporating it into the soil with a studded roller or anchoring it with a stabilizing emulsion.
Structural BMPs Physical, structural, or mechanical devices or facilities intended to prevent pollutants from entering stormwater.
Tailwater Defined as the depth of water downstream of a culvert or other conveyance discharge measured from the outlet invert.
Temporary BMPs
Designed to prevent the introduction of pollutants into runoff for the duration of the construction project and are concurrent with construction of the permanent BMPs. Common examples of temporary BMPs include the mulching of bare ground, silt fencing, and spill control and containment.
GLOSSARY - 7 August 2010
Temporary Erosion and Sediment Control (TESC) Plans Plans that describe the measures used during construction to protect state waters from degradation.
Territorial Seashore Reserve
A distinct and valuable natural resource, existing as a delicately balanced ecosystem, protected by the Territorial Seashore Protection Act.
Turbidity Curtain A fabric barrier used to isolate the near-shore work area.
Vegetated Surface A permanent perennial vegetative cover on areas that have been disturbed.
Wet Pond Constructed basins containing a permanent pool of water throughout the wet season.
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1 - 1 August 2010
1 PURPOSE AND SCOPE 1
The purpose of this Guam Transportation Stormwater Drainage Manual (TSDM) is to 2 provide guidelines for the planning and design of stormwater management facilities for 3 transportation projects on the island of Guam. Transportation projects include any new 4 development, redevelopment, reconstruction, rehabilitation, or restoration of highways, 5 roadways, and parking, including the appurtenant structures such as bridges and culverts. 6 Conformance to the provisions of this TSDM will result in consistent design procedures and 7 should support acceptance of stormwater planning by regulatory agencies. These 8 guidelines take into account the variations in climatic, geologic, and hydrogeologic 9 conditions of the island. 10
This TSDM establishes the guidelines and provides uniform technical criteria for avoiding 11 and mitigating impacts to water resources associated with the development of roadways 12 and the associated infrastructure. This TSDM will be updated regularly to reflect changes in 13 regulations and advances in stormwater management technology, and to clarify issues. 14
The intended users of this TSDM are the Department of Public Work’s (DPW’s) consultants, 15 site planners, engineers, contractors, reviewers, maintenance staff, and construction 16 managers who design and construct transportation drainage systems. 17
The design criteria established in this TSDM supersedes other previously-published 18 stormwater drainage manuals and guidelines used by DPW. 19
Transportation stormwater management requirements for Guam have been developed to 20 help mitigate the effects of hydrologic change and water quality degradation that can occur 21 from project development on receiving waters of Guam and the United States. 22
This TSDM follows a presumptive approach to achieve compliance with federal and local 23 water quality regulations. Projects that follow the stormwater Best Management Practices 24 (BMPs) contained in this TSDM will be better suited to satisfy Guam EPA (GEPA) and 25 federal requirements. This TSDM includes documentation guidelines that will be necessary 26 for the permitting review process. 27
1.1 THE IMPORTANCE OF STORMWATER MANAGEMENT 28
Land development changes the physical, chemical, and biological conditions of waterways 29 and water resources. The addition of buildings, roadways, parking lots, and other 30 impervious surfaces can reduce natural infiltration, increase runoff, and provide additional 31 sources of pollution. 32
Depending on the magnitude of changes to the land surface, the total runoff volume can 33 increase dramatically. These changes can also accelerate the rate at which runoff flows 34 across the land. This effect is further exacerbated by drainage systems such as gutters, 35 storm sewers, and lined channels that are designed to carry runoff to rivers and streams 36 quickly. 37
1 - 2 August 2010
Without proper planning and design, development and impervious surfaces reduce the 1 amount of water that infiltrates into the soil and groundwater, thus reducing the amount of 2 water that can recharge aquifers and feed stream flow during periods of dry weather. 3 Unmanaged stormwater from development and urbanization affects both the quantity and 4 quality of the runoff. Development can increase the concentration and types of pollutants 5 carried by runoff. 6
Linear in nature, transportation projects tend to encompass multiple drainage basins and 7 impact multiple receiving waters. Even though the runoff from highways may be a fraction 8 of the runoff affecting nearby water bodies, it can contribute to the cumulative degradation 9 of those waters unless suitable BMPs are utilized. 10
The construction of roadway projects can contribute to surface runoff contamination, 11 primarily due to suspended solids associated with soil erosion. Without suitable controls, 12 construction activities can also result in the contamination of stormwater that results from 13 vehicle operations and maintenance; use and storage of fuels, solvents and paints; and 14 uncured asphalt and concrete. Those impacts can be severe and long-lasting if appropriate 15 actions are not taken to control construction site runoff quality. 16
1.2 MANAGEMENT OF RUNOFF FROM TRANSPORTATION PROJECTS 17
Federal and Guam regulations call for the implementation of both operational and 18 technology-based BMPs to reduce the discharge of pollutants in stormwater to the 19 maximum extent possible, before discharging back to natural waterways. 20
The application of BMPs is the key to managing stormwater runoff from transportation 21 projects. The term BMP refers to operational activities or physical controls that control flows 22 and velocities and/or reduce pollution that would otherwise impact the receiving water 23 bodies. 24
BMPs identified as temporary and permanent have been classified into four categories, as 25 shown in Table 1-1 below. Pollution prevention BMPs are the incorporation of good 26 engineering practices during design and planning to limit a project’s adverse affect on the 27 natural ground cover and flow patterns. Oftentimes, additional planning in the layout and 28 staging of a project can reduce the disturbed ground area and maintain more natural runoff 29 patterns then what typically might be expected. 30
Runoff treatment BMPs are the application of long-term facilities that control flow rates 31 and/or remove pollutants from runoff. This is typically accomplished by using detention 32 storage and ground infiltration. Other BMPs use gravity settling of particulate matter, 33 vegetative and physical filtration, biological uptake, and soil absorption to remove 34 pollutants. Flow control BMPs reduce the peak rate of runoff during a storm event by either 35 infiltration into the soil profile or by storing the flow and releasing it at a slower rate. This 36 helps to maintain the existing and more natural flow rates to limit erosive effects on 37 downstream waterways. Permanent BMPs are used to treat highway runoff for the design 38 life of the project site. 39
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Construction site BMPs are temporary measures designed to prevent the introduction of 1 pollutants into runoff for the duration of the construction project. These measures also 2 include infiltration, filtration, and particulate settling devices, which are installed and 3 functioning prior to the start of construction. Common examples of temporary BMPs include 4 bare- ground mulching, silt fencing, sedimentation basins, and spill control. 5
Maintenance BMPs are the long-term maintenance of the roadway project area and the 6 associated BMPs to keep them operating at proper efficiencies. These can include such 7 maintenance activities as sweeping and mowing to remove pollutants at the source. Other 8 typical long-term maintenance activities include keeping ponds and storm sewers clean of 9 debris and sediments. This TSDM only discusses maintenance guidelines for stormwater 10 BMPs. General maintenance activities are not listed in detail in this TSDM, because they 11 are the subject of separate DPW guidelines. 12
BMP Description
Design Pollution Prevention BMPs
Minimizing flow and quality impacts by improved design and planning
Runoff Treatment BMPs Permanent quality treatment and flow control devices and facilities
Construction Site BMPs Temporary sediment and waste control during the construction process
Maintenance BMPs Long term regular maintenance of the transportation facilities and associated BMPs
Table 1-1: BMP Classifications 13
1.3 ORGANIZATION OF THIS MANUAL 14
This TSDM is divided into eight chapters for the aid of the designer. Chapter 1 gives 15 background information on the development of this TSDM and an overview of the 16 stormwater problems associated with roadways and other parts of the transportation 17 infrastructure. 18
Chapter 2 provides an overview of the design process and how the stormwater/drainage 19 design elements should be integrated into that process. Guidelines are provided for 20 gathering pre-design data, reviewing design alternatives, and finalizing the selected 21 alternative. 22
Chapter 3 describes the minimum requirements that apply to the planning and design of 23 stormwater facilities and BMPs. Guidelines are provided to determine which of the nine 24 minimum requirements must be met for a given transportation project. The purpose and 25 applicability of the minimum requirements are also described in this chapter. 26
Chapter 4 discusses transportation hydrology; the conversion of rainfall to runoff as needed 27 for conveyance; and BMP engineering purposes. This chapter lists which analysis methods 28 to use, providing the required data and assumptions. 29
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Chapter 5 contains a discussion and guidelines on stormwater hydraulic design. This 1 chapter also contains criteria on the selection, sizing, and documentation of the stormwater 2 conveyance systems. 3
Chapter 6 guides the designer through the process of selecting and designing permanent 4 stormwater quality treatment and flow control BMPs that are applicable for Guam’s 5 transportation conditions. This chapter includes details of the design and construction 6 criteria for each BMP. 7
Chapter 7 includes criteria for selecting appropriate Temporary Erosion and Sediment 8 Control (TESC) measures. Appropriate TESC BMPs are required for all transportation 9 projects and should also be included in the contractor’s stormwater pollution prevention 10 plan, when required. 11
1.4 HOW TO USE THIS MANUAL 12
The designer should follow the guidelines included in Chapter 2 for integrating the planning 13 and design of stormwater-related project elements into the context of the project 14 development process. This process should be completed prior to using the guidelines in 15 Chapter 3 to determine which minimum requirements must be satisfied for a specific 16 project. In most instances, this process will prompt design of the post-construction BMPs 17 according to the criteria provided in Chapters 4, 5, 6, and 7. Most projects lend themselves 18 to a relatively straight-forward application of one or more of the BMP options presented in 19 this TSDM. Finally, while working from a constructability point of view in accordance with 20 the project construction staging plan and schedule, the designer will prepare a TESC plan 21 in accordance with Appendix I. 22
1.5 REVIEW PROCESS AND REGULATORY STANDING OF THIS MANUAL 23
This TSDM provides guidelines for the planning and design of stormwater management 24 facilities for transportation projects on the island of Guam. This TSDM meets the level of 25 stormwater management established by GEPA in its Draft GEPA 2010 Stormwater 26 Management Regulations and the Guam Water Quality Regulations; however, GEPA 27 reserves the right to enforce regulatory compliance as provided for in Guam statutes and 28 regulations. See Appendix IA. 29
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2 STORMWATER PLANNING AND GUAM DRAINAGE POLICIES 1
This chapter provides guidelines for integrating the planning and design of stormwater- 2 related project elements into the context of the Department of Public Work’s (DPW’s) 3 project development process. It also describes various federal and local policies that 4 regulate stormwater discharges. This Guam Transportation Stormwater Drainage Manual 5 (TSDM) makes all attempts to adhere to all current regulations. Because regulations evolve 6 with time, this TSDM will be updated to remain current with regulations. 7
2.1 DRAINAGE MASTER PLANNING AND PROJECT DEVELOPMENT 8
Stormwater master planning is an important tool a designer can use to assess and prioritize 9 both existing and potential future stormwater problems, as well as consider alternative 10 stormwater management solutions. 11
Stormwater planning is often used to address specific single functions such as drainage 12 provisions, flood mitigation, cost-benefit analyses, or risk assessments. Multi-objective 13 stormwater master planning broadens this traditional definition to potentially include land 14 use planning and zoning, water quality, habitat, recreation, and aesthetic considerations. 15
The integration of stormwater planning and design is crucial to DPW’s project development 16 process. How the process applies to a specific project depends on the type, size, and 17 complexity of the project. The project development process consists of the distinct phases 18 described below. In practice, the phases actually overlap, and some design modifications 19 may occur during the construction phase. Consultants may or may not be involved in any 20 phase, but inclusion of this process into this TSDM reinforces DPW’s commitment to 21 integrate stormwater planning and design into project development. 22
Scoping phase: Development of this phase is based on the general project description and 23 early estimates of needs and costs. Under this phase, a number of project alternatives may 24 be evaluated and reviewed. Based on the relative merits of the alternatives, the better 25 alternatives are selected and included in a summary document. The environmental section 26 of the project summary establishes the environmental classification of the project and the 27 level of environmental documentation required for the project. 28
Preliminary design phase: Based on the summaries and cost estimates completed as 29 part of the scoping and programming work, DPW will make a decision as to a selected 30 alternative for additional storm drainage refinement and environmental assessment work to 31 be performed. This is the preliminary design phase during which the stormwater design is 32 further progressed; the Best Management Practices (BMPs) selected; and the 33 environmental assessments completed. The subsequent hydraulic and environmental 34 documentation is reviewed, and upon acceptance, DPW issues a “start work” order for both 35 the permit applications and preparation of the final design and construction bid documents. 36
Environmental permitting and final plans, specifications, and cost estimates (PS&E) 37 phase: The final design and construction cost estimate for the stormwater BMPs is 38
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accomplished during this phase. All environmental commitments are incorporated and the 1 final project permits are obtained, while the final design plans and specifications are 2 finalized and submitted ready for construction bidding. 3
The design and environmental process continues during construction as part of shop 4 drawing reviews, design support for change orders, and Requests for Information (RFIs). 5
DPW should be consulted at each phase of the project development process for reviews 6 and approvals, and to gain feedback regarding any project-specific requirements. Adhering 7 to the minimum requirements and the BMP selection process discussed in this TSDM plays 8 a critical role in the project development process, because it minimizes costly design 9 changes, reduces delays in obtaining permits, and keeps the project in compliance during 10 construction, operation, and maintenance activities. Table 2-1 describes the project 11 development process. 12
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Scoping
Scope Approval
Preliminary Design, Refinement, and Documentation
Concept Approval
Environmental Permitting and Final Design
Preliminary identification of water quality and hydrologic impacts; conveyance and project flow patterns; and potential treatment and flow control BMP alternatives
Selection and refinement of stormwater conveyance and BMP facilities
Provide drainage design support for the environmental assessments
Final design of stormwater facilities
Obtain environmental permits
Project storm drainage summary including:
Conducting preliminary stormwater summary and scope
Providing support for environmental reviews
Identifying additional right-of-way needs
Listing potential utility conflicts
Project design report and environmental permit applications as supported by:
Hydraulic Report including drainage calculations, soils analysis report, percolation test results, property maps, site plans, wetlands delineation map, drainage flow maps with land contours, etc.
Environmental Assessment Reports and Impact Statements as supported by stormwater design and coordination
PS&E package:
Temporary Erosion and Sedimentation Control (TESC) plans (Appendix I)
Provisions for Spill Prevention, Control, and Countermeasures (SPCC) plans
Stormwater plans and details
Specifications and special provisions
Project cost estimate and schedule
Contractual bids, selection of contractor, and start of construction
Cost estimates for stormwater facility design and construction alternatives
Project conceptual (preliminary) plans and cost estimate
Stormwater design support during construction
Table 2-1: Project Stormwater Development Process 1
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2.1.1 SCOPING AND PLANNING 1
This section describes various stormwater planning activities that should be undertaken to 2 scope a project. The assessment and documentation of stormwater impacts and mitigation 3 measures begins during project scoping. Project type, size, and complexity are key factors 4 in the development of the stormwater strategy. 5
2.1.1.1 SITE ASSESSMENT 6
Understanding each site’s drainage patterns is essential to optimizing stormwater design for 7 a project. The design team must identify all natural areas and off-site flows coming to the 8 site, including streams and stormwater discharges. This information is shown on a 9 “drainage map”. The drainage map should be prepared to sufficient detail and scale to 10 show the entire watershed (in particular, the upstream watershed limits) that the project 11 may be intersecting. The map should include the types of ground cover, soil types, 12 elevation contours, flow paths, existing wetlands and streams, existing conveyance, 13 existing cross-culverts, right-of-ways and easements, and existing water quality treatment 14 and flow control facilities. The map is the key planning tool for design of the project 15 stormwater facilities by including the project drainage basins and sub-basins, discharge 16 locations, critical areas, downstream impact locations, and potential BMP sites. The map is 17 also the basis for determining runoff calculation parameters such as times of concentration, 18 drainage areas, curve numbers, and conveyance slopes. 19
Further, as the design is developed, the proposed improvements, drainage conveyance, 20 and BMPs are added to the map. The drainage map becomes the main exhibit in the 21 project’s hydraulic report, which is part of the design and construction record. 22
The transportation facility should allow for the passage of all off-site flows. The designer 23 should work to maintain the existing flow patterns as much as possible. To minimize 24 conveyance and BMP costs, it is generally best to keep off-site flows separate from the 25 highway runoff. Most importantly, runoff from the project should not adversely impact 26 downstream receiving waters and properties. 27
The conservation of natural areas helps to minimize project impacts. Some of these areas 28 may be used as part of the project’s stormwater management approach, especially if they 29 are appropriate areas for dispersion and infiltration. 30
2.1.1.2 GEOTECHNICAL EVALUATIONS 31
Understanding the soils, geology, geologic hazards, and groundwater conditions is the first 32 step to good drainage planning and design. The stormwater designer should provide input 33 as to what is needed in regards to the stormwater planning when the design team scopes 34 the overall geotechnical investigation work. The main items needed for drainage are 35 geotechnical evaluations of geological hazards and groundwater issues, especially where 36 existing problems have been identified. The geotechnical investigations should include soil 37 characteristics in the project vicinity, including estimates of infiltration rates and 38
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groundwater depths at potential stormwater BMP locations. Specific soil investigations will 1 be required for any proposed hydraulic structures, and for new storage or infiltration 2 facilities. 3
Seasonal variations of the groundwater table for infiltration and treatment pond designs 4 require the installation of piezometers. One year of monitoring is desirable, but at a 5 minimum, water level readings should be taken through one rainy season. Critical issues to 6 consider include the following: 7
Depth to water table (including any seasonal variations) 8
Presence of soft or otherwise unstable soils 9
Presence in soils of shallow bedrock or boulders that could adversely affect 10 constructability 11
Presence of existing adjacent facilities that could be adversely affected by 12 construction of the stormwater facilities 13
Presence of geologic hazards such as earthquake faults, abandoned mines, 14 landslides, steep slopes, or rock fall 15
Adequacy of drainage gradient to ensure functionality of the system 16
Potential effects of the proposed facilities on future corridor needs 17
Maintainability of the proposed facilities 18
Potential impacts on adjacent wetlands and other environmentally sensitive areas 19
Presence of hazardous materials in the area 20
Whether or not the proposed stormwater plan will meet the requirements of 21 resource agencies 22
Infiltration capacity (infiltration and percolation rates for project sites) 23
2.1.1.3 RIGHT-OF-WAY 24
Examine the proposed layout of the project, and determine the most suitable sites available 25 to locate the stormwater facilities. Typically, the stormwater designer should be able to fit 26 the required conveyance systems, treatment and detention facilities, and outfalls within the 27 existing right-of-way. However, infiltration and detention ponds may require additional plots 28 outside of DPW’s boundaries. The locations and specifics for additional right-of-way should 29 be established as early as possible, as the acquisition process is lengthy and will contribute 30 heavily to the project’s cost. Often times, the designer is required to provide a legal right-of-31 way survey, drawings, and other support for the property acquisition process. 32
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2.1.1.4 UTILITIES 1
A search of utility company records should be conducted to obtain information about 2 locations and easements of existing utilities (and utilities planned to be installed in the near 3 future) that may be impacted by the project. These records are assembled into an “Existing 4 Utility Plan” for planning and future design use. Potential conflicts must be further evaluated 5 and resolved prior to the start of construction. While it is usually best to try and plan 6 stormwater facilities to avoid conflicts with existing utilities, sometimes the utility must be 7 relocated. Depending on the existing utility agreement with DPW, the relocation may be a 8 project cost or the obligation of the utility company. Often times, relocation involves either 9 establishing or providing new easements for the utility company. Early planning of the 10 drainage to minimize utility impacts will also help to limit the overall project costs and 11 reduce impacts to the schedule. 12
2.1.1.5 MAINTENANCE REVIEW 13
There are several design reviews that should be completed with DPW’s maintenance staff. 14 The first review is usually completed during the preliminary scoping or preliminary design 15 phase to determine locations having specific drainage issues. This review helps to define 16 locations that have frequent flooding, flow over-topping, sediment and debris buildup, 17 groundwater issues, and erosion problems. This is also the time to obtain the maintenance 18 staff’s preferences and procedures with regards to regular maintenance work. 19 Understanding their approach to maintenance work, the type of equipment used, frequency 20 of work, and access requirements will be critical when deciding on locations, types, and 21 features of the selected stormwater facilities. 22
Once the preliminary design for the project is finished, the features of the stormwater 23 facilities should be reviewed with the maintenance staff. Their input will be critical in 24 finalizing the permanent conveyance and treatment systems with regards to ease of access 25 for inspections and cleaning. Maintenance work becomes an overall long-term cost to the 26 project, and design coordination with the maintenance staff will help to minimize those 27 costs. 28
2.1.1.6 ENVIRONMENTAL SUPPORT 29
The thorough documentation of stormwater-related environmental impacts and tracking of 30 stormwater design commitments is a required element of the National Environmental Policy 31 Act (NEPA), as well as other environmental laws and environmental permitting agencies. 32
As a project’s scope is defined by the design team, a parallel effort is made by the 33 environmental specialists to define the project’s impacts on the environment and methods 34 to avoid or otherwise mitigate those impacts. The environmental evaluations will become 35 the basis for the permitting applications and agency reviews. After the initial scope is 36 approved, a more formal environmental assessment is made and documented; 37 environmental commitments are made; and permit applications are prepared. Much of the 38
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environmental assessment is based on the project’s temporary and permanent impacts due 1 to stormwater runoff. 2
The stormwater designer should be prepared to support the project’s environmental 3 specialist team during both the scoping and preliminary design phases, by providing the 4 proposed stormwater configuration layouts, BMP types and locations, and discharge 5 locations. Information typically needed by the environmental team is drainage basin and 6 sub-basin areas; discharge locations for each drainage basin; acres of existing and new 7 impervious surfaces; locations and areas of impacts to wetlands and waterways; area and 8 volume of fill in waterways below the ordinary high water level; areas of new and existing 9 pavement runoff being treated by watershed or drainage basin per outfall location; types of 10 treatment; pavement areas having runoff flow control; pre- versus post-pollution loading 11 calculations; quantities of fill in floodplains; and upstream and downstream hydraulic 12 impacts due to cross-drain and culvert work. The information provided will need to be 13 supported with exhibits and calculations. 14
To limit impacts to critical areas, the stormwater designer should coordinate with the 15 environmental specialist frequently while locating stormwater facilities and outfalls. Early 16 discussions between the designer and specialist may help to avoid later permit issues with 17 regards to such things as bridge and culvert placement, aquatic habitats and fish passage, 18 erosion and sedimentation control, groundwater or potable water well contamination, local 19 vegetative mitigation, and project pollution loadings. 20
Through the final design, the coordination between the environmental specialist and the 21 stormwater designer continues as the designer prepares the TESC plans and designs 22 facilities that meet the environmental permit commitments and requirements. The 23 stormwater designer often supports construction and/or the contractor’s environmental 24 manager in the preparation of water monitoring plans, pollution and spill control plans, and 25 TESC updates. 26
2.1.1.7 STORMWATER SUMMARY 27
Stormwater documentation during the scoping phase of project development is referred to 28 here as the stormwater summary and scope. This package contains the information used to 29 predetermine stormwater impacts and the initial selection of stormwater BMPs. It is the 30 source of stormwater information needed to complete the project summary documents. This 31 package must include a brief summary report containing the following: 32
Identification of the project program 33
Brief project description 34
Synopsis of data gathered during the site assessment 35
Drainage map 36
Basin and sub-basin identification 37
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Discharge area delineations indicating flow paths and outfalls to receiving waters 1
Area determinations 2
Applicable minimum requirements 3
Other applicable regulatory requirements related to stormwater 4
Design criteria required for flow control and runoff treatment 5
Known problems and commitments 6
Design alternatives and assumptions for flow control and runoff treatment 7
Cost estimates 8
The stormwater summary report documents the design efforts and decisions made during 9 the scoping phase of the project’s development, and must be retained and easily-10 retrievable. Once the project is reviewed, alternatives are selected, and the designer is 11 selected by DPW, the file and report become the starting point for the preliminary design 12 phase. Note that the report is internal to DPW and its consultants, and serves as a starting 13 point for preliminary design. 14
2.1.2 PRELIMINARY DESIGN 15
The preliminary design develops the preferred stormwater drainage alternatives 16 (alternatives that were identified in the scoping phase) into a workable concept with enough 17 detail to enable preparation of cost estimates. As part of the preliminary design process, 18 data is developed to support the parallel environmental assessment and permit application 19 preparation efforts. 20
2.1.2.1 HYDRAULIC REPORT 21
The hydraulic report is intended to serve as a complete record containing the engineering 22 justification for all drainage design and design modifications that occur as a result of project 23 construction. The report documents the pre-project existing hydraulic conditions, as well as 24 the post-project stormwater management facilities. The report contains all of the designer’s 25 intent, criteria, and calculations for the stormwater design. The hydraulic report is the basis 26 for approval of the project drainage facilities by DPW. This report is then updated during 27 construction, with any modifications, to act as a permanent as-built record of the project’s 28 stormwater management facilities. Appendix II contains detailed instructions and a sample 29 outline required for preparation of the hydraulic report. 30
During the preliminary design phase, a draft (or preliminary) hydraulic report is prepared. 31 This preliminary hydraulic report is an expansion of the stormwater summary report 32 developed as part of the scoping phase. The preliminary hydraulic report contains the 33 existing condition descriptions and the layout and general support calculations for the 34 selected stormwater management alternatives. At this stage of the project development, the 35 stormwater flows, general conveyance routing, and BMP types and sizes are finalized, from 36
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which the cost estimates and right-of-way requirements can be determined. However, the 1 preliminary report lacks many of the final sizing and design details prepared during the final 2 design phase. The preliminary hydraulic report must be reviewed and approved by DPW 3 prior to start of the final stormwater design. 4
The final hydraulic report is a detailed compilation of the final stormwater management 5 design. This final report contains detailed descriptions and calculations for all hydraulic 6 design assumptions and decisions. It also includes the development of the TESC plans for 7 construction, as well as the permanent post-development facilities. 8
If any changes are made to the final stormwater design during construction, the changes 9 are documented with all supporting calculations, plans, and exhibits as attachments to the 10 hydraulic report. Major revisions require separate documentation and approval by DPW. 11 Minor revisions are assembled and included as a final attachment to the hydraulic report as 12 an as-built record of the project’s stormwater management development. 13
2.1.3 FINAL DESIGN 14
The final design of the project follows the approval of the preliminary design, cost 15 estimates, and environmental documentation by DPW. The project’s permit applications 16 and final design preparation is usually completed on a parallel schedule, typically taking 17 approximately the same amount of time. The final design work can proceed by two different 18 procedures in accordance with DPW’s direction and according to the type and suitability of 19 the project. The first method is a final design prepared for the traditional design-bid-build 20 method of project implementation. The second is a final design prepared for a design-build 21 project approach. 22
2.1.3.1 DESIGN-BID-BUILD 23
In the design-bid-build method, the designer develops the preliminary plans into a detailed 24 set of plans and specifications ready for contractor bidding. The stormwater management 25 facility designs are brought up to a level of plan and detail that describes the specific 26 requirements the contractor must meet with regard to layout, levels, grades, and materials. 27 While not telling the contractor exactly how to conduct its operations, the plans, details, and 28 specifications describe as precisely as possible the level of performance and testing 29 standards that must be met. As part of the process, the designer also prepares a 30 construction schedule and cost estimate for DPW’s use to assist in defining the contractual 31 time limit to complete the work, and to compare against the contractor bids. 32
Although not part of the contractual bid package, the stormwater designer must prepare a 33 final hydraulic report. The designer will obtain DPW’s final approval of the stormwater 34 facility plans and supporting calculations, accomplished through review and approval of the 35 final hydraulic report. This report confirms that DPW’s drainage criteria, environmental 36 commitments, and permit requirements have been met. 37
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The stormwater designer is usually responsible for a number of elements in the final project 1 design package. These normally include: 2
Drainage plans: Show proposed roadway alignment and edge of pavement; existing 3 stormwater system including proposed conveyance structures, pipes, ditches, and 4 channels; and proposed stormwater quality treatment and flow control facilities. 5 Each structure and pipe is uniquely numbered for reference in the quantities tables 6 and profile detail sheets. Deletions of the existing systems and other construction 7 constraints are shown by notes. 8
Drainage profile details: The details for installation of the stormwater conveyance 9 facilities are shown on pipe profile sheets. Each structure, ditch, and pipe is shown 10 in profile, with existing and proposed ground surfaces; layout station and offset; 11 type; size or dimension; pipe length; slope; and any specific requirements such as 12 rock protection at outlets, unique connection collars, headwalls, type of grate or 13 cover, connection to existing, etc. Each pipe, ditch, and structure is identified by its 14 corresponding number on the plan sheets. 15
Drainage details: Most drainage structural and installation details are contained in 16 DPW’s Standard Drawings. These are referred to on the profile sheets and by 17 reference in the contract documents. However, most projects will require unique 18 hydraulic structure designs requiring specifically-detailed design sheets. Other items 19 requiring more detail for construction are items such as stormwater treatment and 20 flow control BMPs, where each facility requires a unique design to fit the site. 21
Quantity sheets: The contractual plan set includes a listing of pipes, structures, and 22 earthwork in tablature. Each item is identified by its corresponding number on the 23 plan sheets. The stormwater designer is responsible for assembling a detailed 24 listing of stormwater structure, pipe, and earthwork quantities. 25
TESC plans: The stormwater designer prepares a detailed TESC plan set for the 26 project. The plan includes details for controlling and minimizing sediment in the 27 stormwater runoff during construction for each major construction phase. The plans 28 are prepared in close coordination with the project schedule and staging designers. 29 These plans are sometimes prepared earlier, in support of the environmental permit 30 application review process. After award of the contract, the contractor will modify 31 these plans as applicable to fit its specific staging and work elements. Appendix I 32 contains detailed instructions and a sample outline required for preparation of the 33 TESC plans. 34
Stormwater Pollution Prevention Plans (SWPPP): The SWPPP usually consists of 35 the TESC and spill prevention, control and countermeasures (SPCC) plans. The 36 details of SWPPP are usually a requirement of the contractor to produce. However, 37 most plan and specification contract packages will have an example plans or details 38 of what will be required. 39
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Specifications: Most of the performance and other technical standards are included 1 in DPW’s Standard Specifications. However, there are always unique elements in 2 every project that require a new specification be prepared. These are called Special 3 Provisions, and are written in a standard format that includes the unique item 4 description, performance requirements (materials and installation), and 5 measurement and payment methods. 6
Bid list: The project items to be constructed are summarized by type in accordance 7 with the appropriate specification pay item description. The quantities are 8 summarized for the contractor to enter its bids into the tablature format. The 9 stormwater designer is responsible for assembling the stormwater and TESC items 10 and quantities. 11
Construction cost estimate: The stormwater designer will prepare a construction 12 cost estimate based on the quantity of each pay item in the project. Each item is 13 assigned a unit price estimate of materials and installation cost by a contractor. The 14 pricing of items is based on experience with past project and construction methods, 15 using prices from other recent bids by DPW or other similar projects nationally. This 16 cost estimate is for DPW’s use and is kept confidential until after the contractual 17 bids are opened. 18
Construction schedule: The stormwater designer will provide input as to how the 19 project can be staged during construction, with estimates on how long it takes to 20 perform the work, for an overall estimated construction schedule. This schedule 21 becomes the basis for the contractual time length and for DPW’s monitoring of the 22 contractor’s work. 23
The stormwater designer will provide support during the contractual bidding process to 24 answer any stormwater questions by the bidders and to further review any required bid 25 submittals after bid opening in regard to contractor qualifications and procedures that 26 contribute to the final selection of a contractor. 27
The stormwater designer is usually required to support the construction management 28 process by responding to the contractor’s or DPW’s RFI on the design, and help to prepare 29 any required change orders that pertain to drainage work. The designer should also finalize 30 any modifications to the drainage design and the as-built drawings as an amendment 31 (attachment) to the final hydraulic report. 32
2.1.3.2 DESIGN-BUILD 33
For the design-build project delivery method, the designer assembles the preliminary plans 34 (often called concept plans) and prepares a set of performance specifications. The final 35 design is the responsibility of the design-build contracting company. DPW maintains control 36 of the performance specifications for the design by only allowing the design-builder to make 37 changes from the conceptual plans to within limited performance-type boundaries. 38
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As part of the design-build process, the bidding process requires each design-build 1 contractor to further develop the conceptual drawings with additional design, such that they 2 can develop a lump sum bid .The uniqueness of this process is that the contractor can 3 develop its own design (within specified limits) to match its own methodologies and 4 scheduling, to ultimately obtain the best price possible. Usually, selection of the best 5 design-build contractor is based on a combination of the lump sum price and a rating 6 system of the proposed methods and schedules to perform the work. 7
The stormwater designer will support the bidding process by answering the design-build 8 contractor’s questions. The designer will also review the various proposals for conformance 9 to the specifications and rate the methods and procedures of the drainage work for 10 comparison and contractor selection. 11
It will be the design-builder’s (usually contractors partnered with design consultants) 12 responsibility to prepare the final design and specifications for construction. For stormwater 13 drainage, this includes the calculations and supporting documentation as a final hydraulic 14 report. Typically, DPW will review the design-builder’s designs at the preliminary, final, and 15 released for construction stages. The designs are generally only accepted with comments, 16 and not approved until the final handover of all project requirements. However, the design is 17 usually performed in close coordination with DPW’s project management staff during over-18 the-shoulder type reviews. Depending on DPW’s project management organization, the 19 stormwater designer will typically be involved in reviews of the design-builder’s stormwater 20 submittals. 21
2.2 SPECIAL DESIGN CONSIDERATIONS 22
2.2.1 CRITICAL AND SENSITIVE AREAS 23
Critical areas are defined as wetlands (including streams and lakes), floodplains, shorelines 24 and reefs, aquifer recharge areas, sink holes, well heads, geologically hazardous areas, 25 and those areas necessary for fish and wildlife conservation. 26
2.2.2 WETLANDS 27
Altering land cover and natural drainage patterns may alter stormwater input into 28 surrounding wetlands. Hydrologic changes have more immediate and greater effects on the 29 composition of vegetation and amphibian communities than do other environmental 30 changes, including water quality degradation. Wetlands are protected under federal law and 31 any project encroachment or impacts to the wetlands and their buffers must be mitigated. 32 Project stormwater facilities should attempt to maintain the same flow patterns, rates, and 33 quality of runoff to the wetlands as the existing conditions. 34
Wetland ecosystems are highly effective managers of stormwater runoff. They effectively 35 and naturally remove pollutants, attenuate flows, and recharge groundwater. Minimum 36 Requirement 7 (see Chapter 3) addresses wetland protection. Note that natural wetlands 37
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cannot be used for temporary or permanent project stormwater quality treatment and flow 1 control facilities as required under the minimum requirements in Chapter 3. 2
All projects with the potential to adversely affect wetlands should be evaluated by biologists 3 who assess possible impacts and provide guidance to design teams. The evaluation should 4 include a determination of each wetland's classification, characteristics, quality, and 5 functions. Wetland boundaries are delineated and surveyed when they are close to or 6 overlap with project work areas. The surveyed wetland boundaries should be included on 7 the plans for the project. 8
Upon completion of the field review, a wetland report is prepared for each project, 9 documenting the information gathered on wetlands in the project area, and summarizing the 10 nature and extent of project impacts. 11
The design professionals should follow strict protocols requiring avoidance of all wetland 12 impacts or, where avoidance is not practical, minimization to the greatest extent practical. 13 Special emphasis should be placed on avoiding impacts to high-quality wetlands. When the 14 objectives of a transportation project cannot be met without adverse impacts to wetlands, a 15 wetland mitigation plan detailing how lost wetland functions will be compensated should be 16 prepared. Subsequently, wetland mitigation plans are submitted to one or more regulatory 17 agencies, typically the U.S. Army Corps of Engineers (USACE) and local governments, for 18 their review and permit approval. Even when the impacts are insignificant enough to fall 19 below regulatory thresholds, the department follows a "no-net-loss" directive requiring 20 compensatory mitigation for any wetland loss. 21
2.2.3 FLOODPLAINS 22
Hydraulic flood plain storage that is displaced by roadway fill or other structures will result in 23 a corresponding increase in floodplain water surface levels. This increase may also change 24 wetland hydrodynamics; cause upstream property damage; increase downstream stream 25 flows; and produce additional channel erosion, downstream flooding, and decreased 26 infiltration and summer base flows. Projects may be required to mitigate the loss of 27 hydraulic storage by creating new storage elsewhere within the floodplain. 28
Stormwater discharge from the project area’s new impervious surfaces or changed 29 discharge patterns may also impact the floodplain water surface levels. As part of the 30 downstream analysis, impacts should be checked and mitigated. Refer to the requirements 31 for Minimum Requirement Number 4 in Chapter 3. 32
Stormwater quality treatment and flow control facilities should be placed above the 50-year 33 floodplain level. Stormwater quality treatment facilities should be designed such that flood 34 levels cannot flush the previously-removed pollutants back into the natural water bodies. It 35 is also pointless to install detention facilities that reduce the floodplain storage volume, 36 which must be mitigated elsewhere. 37
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2.2.4 AQUIFERS AND WELLHEADS 1
In general, it is desirable to recharge the underlying limestone bedrock aquifer in the 2 northern Guam area. However, roadway runoff must be pre-treated prior to infiltration into 3 the aquifer. The infiltration BMP description protects the aquifer by requiring pre-treatment 4 prior to the infiltration facility, with the infiltration being through at least 2 ft. of soil depth 5 above the limestone bedrock. The soil interaction with the stormwater provides entrapment, 6 oxidation, and vegetative absorption of the stormwater pollutants. Where less than 2 ft. of 7 soil exists above the limestone bedrock or the groundwater table, the runoff must pass 8 through a prescribed quality treatment BMP prior to infiltration. Likewise, any project 9 discharge to a limestone area sinkhole must be quality treated using an appropriate BMP. 10
The project should be of sufficient distance or provide barriers for physical protection of well 11 heads and limestone area sinkholes. Untreated stormwater from the project area should not 12 be allowed to discharge or infiltrate closer than 1000 ft. to a well. 13
2.2.5 STREAMS AND RIPARIAN AREAS 14
Similar to protection of wetlands, the project’s environmental specialist will establish the 15 boundaries of the stream and its buffer. The stream boundary is usually identified by its 16 Ordinary High Water Level (OHWL or OHW), which is determined by field observation of 17 the vegetative and flow-level indications by the environmental specialist. 18
Similar to the wetlands, the streams are protected by federal law and any project 19 encroachments and impacts to stream channels and their boundaries must be mitigated. 20 Where at all possible, avoid any filling within the OHWL boundaries. For roadway 21 widenings, often times culvert extensions can be avoided by using headwalls, with no need 22 for embankment fill below the OHWL. Other measures to minimize the project’s impacts is 23 using flow spreaders on stormwater outlets for a more natural sheet flow across the stream 24 bank and buffer area into the water body. DPW also prefers using the vegetative erosion 25 protection measures, such as tree stakes and grasses, as a more sensitive alternative to 26 structures or riprap. 27
The design professionals should follow strict protocols requiring avoidance of all riparian 28 impacts or, where avoidance is not practical, minimization to the greatest extent practical. 29 When the objectives of a transportation project cannot be met without adverse impacts to 30 stream and riparian habitats, a mitigation plan detailing how lost riparian functions will be 31 compensated should be prepared. Subsequently, stream and riparian mitigation plans are 32 submitted to one or more regulatory agencies, typically the USACE and local governments, 33 for their review and permit approval. 34
2.2.6 ENDANGERED SPECIES 35
A biological evaluation or biological assessment must be prepared whenever it is suspected 36 that Endangered Species Act (ESA)-listed species inhabit the vicinity of a project. The 37 scoping team must contact the biologist early in the scoping process to request assistance 38
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in determining ESA-related issues and how these issues and needs affect project design 1 and cost considerations. 2
2.2.7 AIRPORTS 3
Special consideration must be given to the design of stormwater facilities for projects 4 located near airports. Roadside features, including standing water (such as wet ponds) and 5 certain types of vegetation, can attract birds both directly and indirectly. The presence of 6 large numbers of birds near airports creates hazards for airport operations and must be 7 avoided. Before planning and designing facilities for a project near an airport, contact the 8 airport and the Federal Aviation Administration (FAA) for wildlife management manuals and 9 other site-specific criteria. 10
2.2.8 BRIDGES 11
Because the over-water portion of the bridge surface captures only the portion of rainfall 12 that otherwise would fall directly into the receiving water body, that portion of the bridge 13 makes no contribution to the increased rate of discharge associated with surface runoff to 14 the water body. 15
Bridges are typically so close to receiving waters that it is often difficult to find sufficient 16 elevations or areas in which to site a stormwater quality treatment solution. In the past, 17 bridges were constructed with small bridge drains that discharged the runoff directly into the 18 receiving waters by way of downspouts. Since this practice is no longer allowed, it becomes 19 challenging to incorporate runoff collection, conveyance, and treatment facilities into the 20 project design. 21
Sometimes the treatment can be completed by treating runoff from another equivalent 22 pavement area that drains into the same waterway. 23
The use of suspended pipe systems to convey bridge runoff must be avoided whenever 24 possible because these systems have a tendency to become plugged with debris and are 25 difficult to clean. The preferred method of conveyance is to hold the runoff on the bridge 26 surface and intercept it at the ends of the bridge with larger inlets. This method requires 27 adequate shoulder width to accommodate flows so that they do not spread farther into the 28 traveled way than allowed. In cases where a closed system must be used, it is 29 recommended that bridge drain openings and pipe diameters be larger, and that 90-degree 30 bends be avoided to ensure the system’s operational integrity. Early coordination with 31 DPW’s maintenance staff is essential if a closed system is being considered. 32
2.3 OVERVIEW OF FEDERAL, TERRITORIAL, AND LOCAL ORDINANCES 33
The list of laws, regulations, and ordinances below are provided only for guidance. It should 34 not be read as a complete list of applicable regulations. Regulations are evolving on a 35 continuous basis. The reader should contact individual agencies for permit requirements. 36
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FEDERAL ACTS 1
Federal Clean Water Act (CWA) 2
The objective of the CWA is to restore and maintain the chemical, physical, and biological 3 integrity of the nation’s waters. It established broad and comprehensive rules, regulations, 4 and authorities for the protection, maintenance, and improvement of water quality for all 5 waters of the United States; and it established specific actions to carry out necessary water 6 pollution prevention efforts by federal and state governments. The applicable sections are: 7
§104 - Authorizes the U.S. Environmental Protection Agency (EPA) to establish 8 programs for the prevention, reduction, and elimination of pollution. 9
§303 - Requires states to establish Water Quality Standards (WQS) and identify 10 those waters that do not comply with standards. 11
§304 - Requires the U.S. EPA to issue guidance to states for identifying and 12 evaluating the extent of Non-Point Source Pollution (NPS) pollutants and methods 13 to control NPS discharges from agriculture, silviculture, construction, and 14 hydromodification. 15
§305 - Requires states to submit biennial reports describing the quality of all 16 navigable waters within the state, including a description of the nature and extent of 17 the NPS source of pollution and recommendations to control such sources. 18
§314 - Requires federal government entities to comply with local requirements to the 19 same extent as any nongovernmental entity. 20
§401 - Requires any applicant for a federal license or permit conducting an activity 21 that may cause a discharge to navigable waters to acquire a water quality 22 certification from the state indicating that such discharges will not violate the state’s 23 WQS. 24
§402 - National Pollutant Discharge Elimination System (NPDES) permits – Require 25 federal permits for point source dischargers. 26
§404 - Requires individuals/entities conducting construction activities within all 27 jurisdictional waters to acquire a Section 404 permit from the USACE and a 28 Section 401 water quality certification from the state. 29
Federal Safe Drinking Water Act (SDWA) 30
§1428 - Requires states to create a Wellhead Protection Program (WHPP). This is 31 an effective means for local governments to protect and manage groundwater 32 resources. Preventing groundwater from becoming contaminated protects the public 33 health and the environment, and is cost-effective. 34
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§1424 - Allows for the designation of principal or sole-source aquifers. In 1978, the 1 groundwater lens of northern Guam was defined as a “principal source aquifer” by 2 the U.S. EPA. 3
Federal Coastal Zone Act - Reauthorization Amendments of 1990 4
§6217 - Led to the establishment of the Guam Non-Point Source Pollution 5 Management Plan, which has received conditional federal approval. This plan is a 6 coordinating document with the goal of controlling NPS through the use of various 7 management measures, and should be adopted and implemented with legal 8 authority. Oversight is provided by the U.S. EPA and the National Oceanic and 9 Atmospheric Administration (NOAA). 10
Federal Comprehensive Environmental Response, Compensation, and Liability Act 11 (CERCLA) 12
CERCLA regulates the investigation and cleanup strategies of hazardous materials 13 and groundwater contamination resulting from military activities. 14
Federal Resource Conservation Recovery Act (RCRA) 15
This act includes hazardous waste and underground storage tank provisions (see 16 40CFR, Part 280, for relevant regulations). 17
TITLE 10 OF THE GUAM CODE ANNOTATED 18
10 GCA, Chapter 46. Guam Water Resources Conservation Act (WRCA) 19
WRCA requires the conservation and beneficial use of all surface and underground water 20 resources; the management of such resources to prevent operation modifications, over-21 pumping by maintenance, abandonment, and destruction; and the avoidance of 22 contamination of water wells as a result of extraction. WRCA established all water 23 resources as the property of the people of Guam. 24
Controls the drilling and operation of wells 25
Authorizes the Groundwater Management Program 26
10 GCA, Chapter 47, Water Pollution Control Act 27
This requires that the Government of Guam conserve, protect, maintain, and improve the 28 quality and potability of public water supplies for the propagation of wildlife, fish, and 29 aquatic life, and for agricultural, industrial, recreational, and other beneficial uses through 30 the prevention, abatement, and control of new or existing water pollution sources. 31
Guam’s soil erosion and sediment control regulations 32
Guam’s feedlot waste management regulations 33
Guam’s WQS 34
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10 GCA, Chapter 48, Toilet Facilities and Sewage Disposal 1
Sewer connection regulations 2
Individual wastewater system regulations 3
10 GCA, Chapter 49, Air Pollution Control Act 4
Air pollution standards and implementation plan 5
10 GCA, Chapter 50, Guam Pesticide Act 6
Guam pesticide regulations 7
10GCA, Chapter 53, Safe Drinking Water Act (SDWA) 8
Guam’s SDWA. 9
§7289(f) - Underground Injection Control Regulations and Program, which protects 10 underground sources of drinking water from contamination. 11
Guam’s WHPP, approved in 1993, provides for three levels of protection: 1) the 12 1,000-foot radius around each drinking water production well; 2) the groundwater 13 protection zone; and 3) the whole northern Guam lens. 14
10 GCA, Chapter 51, Solid Waste Management and Litter Control 15
Solid waste and hazardous waste regulations 16
TITLE 21 OF THE GUAM CODE ANNOTATED 17
21 GCA, Chapter 61, Zoning Act 18
The Guam Territorial Land Use Commission (TLUC) and Application Review 19 Committee (ARC), under the Department of Land Management, review all projects 20 with respect to the 1996 Guam Zoning Law and Regulations. 21
21 GCA, Chapter 61, Subdivision Act 22
The TLUC and ARC, under the Department of Land Management, review all 23 projects with respect to the 1997 Guam Subdivision Rules and Regulations. 24
OTHER PUBLIC LAWS AND EXECUTIVE ORDERS 25
Protection of Wetlands - Guam Executive Order 90-13 26
This executive order declares that the National Wetlands Inventory Map, produced by the 27 U.S. Fish and Wildlife Service, serves as the official, interim wetland map for Guam, and 28 that all government agencies use the map in the review of physical development projects. It 29 further directs the appropriate government agencies including the Guam Environmental 30 Protection Agency (GEPA), the Department of Agriculture, and the Bureau of Planning, to 31 complete a study of wetlands; prepare public information; and draft necessary legislation, 32 rules, and regulations, and/or an executive order to protect wetland resources, including 33
2 - 19 August 2010
water quality and wildlife habitat. During 1999, GEPA released a contract to complete this 1 mandate. 2
Territorial Seashore Protection Act, Public Law 12-108, 1974 3
This declares that the Territorial Seashore Reserve is a distinct and valuable natural 4 resource, existing as a delicately-balanced ecosystem, and that the permanent protection of 5 natural, scenic, and historical resources in the reserve is of paramount importance. This act 6 specifies four primary actions that must be accomplished: 1) studying the seashore reserve 7 to determine the ecological planning principals required to ensure conservation; 2) 8 preparing a comprehensive and enforceable plan based on the study for the long-range 9 conservation, management, and development of the reserve; 3) ensuring interim 10 development will be consistent with this law; and 4) mandating the Board of Directors of the 11 Territorial Seashore Protection Commission to implement the provisions of the law. The 12 Bureau of Planning recently provided funds to the Department of Land Management to 13 complete the plan. The plan will mandate guidelines for approving each development permit 14 for Guam’s seashore reserve areas, regardless of zoning. (Seashore reserves include all 15 land between the 10-fathom contour seaward and 100 meters inland from the mean high-16 water mark.) It will include development setbacks for erosion control and allowable uses 17 that will minimize impacts. 18
Watershed Protection Executive Order - Guam Executive Order 99-09 19
This order affirms the Water Planning Committee’s (WPC) work on watersheds; provides 20 emphasis and direction for agency directors to participate in this important endeavor; 21 emphasizes that watershed protection should be approached from a multiple-ownership 22 and use perspective; directs GEPA to review the statutory fit and applicability of the 23 watershed approach locally; and directs appropriate data management. 24
Interim adoption of Stormwater Management Criteria for Department of Public Works 25 and Other Government of Guam Projects- Guam Executive Order -2005-35 26
This order affirms adoption of CNMI and Guam Stormwater Management Criteria in interim 27 for all Government of Guam projects, to include new roads and road repairs. It also directs 28 DPW to use the new criteria to design stormwater management systems. The interim 29 adoption precludes private development from adoption of the new criteria. 30
2.4 DRAINAGE SUBMITTALS AND REVIEW PROCEDURES 31
A drainage review is the evaluation by DPW’s staff of a proposed project’s compliance with 32 the drainage requirements of this TSDM. All DPW projects that have the following minimum 33 characteristics require drainage design submittals in accordance with Section 2.1: 34
Disturbs 5,000 sq. ft. or more of land 35
Located within the limestone bedrock aquifer recharge area of northern Guam 36
Disturbs any existing drainage facilities 37
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Any project that requires additional stormwater facilities to meet treatment criteria for 1 at least 40 percent of the impervious surface 2
Have a culvert replacement or extension 3
Have previously-identified drainage or groundwater problems 4
Located adjacent to a natural water body, shoreline, wetland, or stream 5
Located within a floodplain 6
Reviews and Approvals 7
Reviews performed by DPW are required for stormwater management alternatives 8 identified during the project scoping phase. Approval will be required before proceeding to 9 the preliminary or concept development phase. DPW’s approval of the selected alternatives 10 and the concept designs should be obtained before proceeding with final designs and 11 contractual documents. The final drainage design plans must be reviewed and approved by 12 DPW prior to advertising the construction document package for bids. 13
DPW’s approval does not supersede the designer’s obligations to perform within the ethics 14 and standards normally acceptable for the engineering profession. The designer is also 15 obligated to prepare the work to project permit commitments and requirements, federal 16 laws, and local ordinances and regulations. 17
2.5 SUBMITTAL REQUIREMENTS FOR DRAINAGE REVIEW 18
2.5.1 QUALIFICATIONS OF ENGINEERS 19
The project stormwater designs should be prepared directly by or under the direction of a 20 graduate engineer having direct experience in stormwater management design. The 21 engineer should be a Professional Engineer with a current Territory of Guam license. All 22 plans, specifications, reports, calculations, and other submittals that will become part of the 23 permanent record of the project must be dated and bear the Professional Engineer’s official 24 seal and signature. 25
2.5.2 HYDRAULIC REPORT 26
All drainage designs should have a hydraulic report. The hydraulic report will be the basis 27 for DPW’s review and approval of the project’s stormwater management facilities. The 28 hydraulic report is intended to serve as a complete documented record containing the 29 engineering justifications for all drainage modifications that occur as result of the project. 30 See Appendix II for additional requirements and an outline of the hydraulic report. 31
2.5.3 PLAN COMPONENTS 32
This section defines the general requirements for stormwater plans, reports, and other 33 documents submitted to DPW for review. 34
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Each submittal to DPW for review should be accompanied by a cover letter stating 1 the purpose of the submittal; a list of the attachments; the project identification titles 2 and numbers; the requested action; the authority of the person making the 3 submittal; and the time limit for review comments. Submittals should be made in 4 both hard copy and electronic format. 5
Each design submittal should be made in standard DPW formats, with a title page 6 and index. 7
Each design submittal should include a vicinity map. The vicinity map should show 8 the project’s general location on Guam, with a blow-up to further define the location 9 by showing the village name, adjacent streets and roads, and recognized local 10 landmarks. An additional blow up to scale should show the project’s limits, with 11 enough detail so that beginning and end stations and main project features can be 12 identified in a site visit. For drainage design submittals, the main watercourses, 13 drainage basins, and water bodies should be shown. 14
Depending on the nature of the type of report or design submittal, reports should 15 start with either a description and purpose section, or with an executive summary 16 stating the purpose, problems, proposal, and conclusions. Reports should be 17 prepared with clear and precise wording, further supported with exhibits, graphs, 18 charts, and tables, as much as possible. Reports should be formatted to 8.5 x 11-19 inch paper size with fold out exhibits on 11 x 17-inch size as necessary. Hardcopies 20 should be neatly bound with a protective cover. 21
Design plans and details should be submitted in an 11 x 17-inch paper format with 22 scale and text that allows for easy reading. The design plans should be prepared to 23 DPW’s standard formats with regard to borders, layout, and components. DPW 24 CADD preferences can be supplied. All layout plans should include a north arrow, a 25 bar scale, and identifying features to locate the sheet within the plan set and the 26 proposed components by survey on the site. Details and sections should be clearly 27 identified as to what view they are and where the section is taken. 28
If a final design submittal, the cover of reports and specifications should be signed 29 and sealed by the Engineer of Record. Each final design and detail drawing sheet 30 should be signed and sealed by the Engineer of Record for that discipline’s work. If 31 a draft submittal, then each sheet should be marked “draft” in large letters on the 32 cover and in the header or footer of each following page. Draft design plans and 33 detail drawings should be marked “draft - not for construction” in prominent lettering 34 on each sheet. All project plan and detail design sheets should be prepared using a 35 recent version of MicroStation or AutoCAD compatible with DPW’s CADD systems. 36
All designs should use English standard units. Tables, graphs, scales, and other 37 grids should be divisible by ten. 38
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3 MINIMUM REQUIREMENTS 1
3.1 INTRODUCTION 2
Each of the Department of Public Work’s (DPW’s) roadway development projects should be 3 reviewed against the Minimum Requirements listed in this chapter. These Minimum 4 Requirements are intended to achieve compliance with federal and Guam water quality 5 regulations, while catering to the unique linear features of most transportation projects. 6 There are nine Minimum Requirements, as follows: 7
1) Stormwater planning 8
2) Construction stormwater pollution prevention 9
3) Source control of pollutants 10
4) Maintenance of natural drainage 11
5) Runoff treatment 12
6) Flow control 13
7) Wetlands protection 14
8) Watershed-based planning 15
9) Operation and maintenance 16
All of the minimum requirements apply to every project, unless the project is exempted (see 17 Section 3.1.1 below). The minimum requirements apply to the entire project area or limits. 18 The project limits are defined as the length of the project and the width of the right-of-way. 19
3.1.1 EXEMPTIONS 20
The Permitting Authority can allow exemptions. The following activities may be exempted 21 from meeting the minimum requirements if the authority determines that the scope and size 22 of the activity will not create either erosion or other hazards to Guam’s surface waters or 23 marine waters, and reports that determination in writing: 24
Pothole and square-cut patching 25
Disturbs less than 5,000 sq. ft. of land 26
Cleaning and general maintenance of stormwater conveyance, quality treatment, 27 and flow control facilities 28
Overlaying or replacing the existing impervious roadway and parking surfaces 29 without expanding the area of coverage, provided stormwater management facilities 30 already exist that meet the requirements of this TSDM 31
Edge of pavement (shoulder) grading/shaping 32
Roadway surface crack sealing 33
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Vegetation maintenance 1
Exploratory excavations for the purpose of soils testing, provided that no grubbing or 2 grading is to be performed 3
Clearing of land for the purpose of making topographic surveys and hand-clearing 4 trails for survey lines and access for soil exploration equipment 5
3.2 MINIMUM REQUIREMENTS 6
This Section describes the Minimum Requirements that must be met by non-exempt 7 transportation projects. 8
3.2.1 MINIMUM REQUIREMENT 1: STORMWATER PLANNING 9
See Chapter 2, Section 2.1 for the details of incorporating stormwater planning into project 10 development. The integration of stormwater planning and design is crucial to DPW’s project 11 development process. 12
All non-exempt DPW transportation projects will require stormwater planning and a positive 13 method to deal with the stormwater within the project limits. Project area runoff must be 14 contained within the project limits, treated, detained, and either infiltrated or conveyed for 15 discharge to a natural water body (stream, river, wetland, tidal water, pond, or lake). Prior to 16 infiltration or discharge, the project area stormwater may be subject to additional treatment 17 and flow control requirements, as discussed in these Minimum Requirements. Project 18 stormwater is not allowed to flow off site to damage or flood neighboring properties. 19
3.2.1.1 GUIDELINES 20
The basic steps involved in stormwater planning are as follows: 21
STEP 1- Collect and analyze information on existing conditions. 22
STEP 2- Prepare a drainage map with conceptual layout. 23
STEP 3- Perform an off-site analysis. 24
STEP 4- Prepare a permanent stormwater management plan. 25
STEP 5- Prepare a temporary erosion and sedimentation control plan. This plan will 26 be incorporated into the contractors’ Construction Stormwater Pollution 27 Prevention Plan (SWPPP). 28
STEP 6- Prepare a hydraulic report. 29
STEP 7- Check compliance with all applicable minimum requirements. 30
STEP 1 – Collect and Analyze Information on Existing Conditions 31
Collect and review information on existing site conditions including topography; drainage 32 patterns; soils; ground cover; and the presence of any critical areas, adjacent areas, 33
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existing development, existing stormwater facilities, and adjacent on- and off-site utilities. 1 Then analyze the data to determine the site limitations, including: 2
Areas with a high potential for erosion and sediment deposition (based on soil 3 properties, slope, etc.) 4
Locations of sensitive and critical areas (e.g., vegetative buffers, wetlands, steep 5 slopes, floodplains, geologic hazard areas, streams, etc.) 6
Prepare an Existing Conditions Summary that will be submitted as part of the Hydraulic 7 Report. The information collected in this step should be transferred to the drainage map 8 prepared as part of Step 2. 9
STEP 2 – Prepare a Preliminary Project Layout and Drainage Map 10
Based on the analysis of existing site conditions, locate the topographic features for the 11 proposed project and prepare a conceptual stormwater layout. 12
Fit the project to the terrain to minimize land disturbance; and confine construction 13 activities to the least area necessary and away from critical areas. 14
Define drainage basins that the project intersects. Define on-site sub-basins and 15 points of discharge. 16
Preserve areas with natural vegetation, especially forested areas, as much as 17 possible. 18
Minimize impervious areas. 19
Maintain and utilize the natural drainage patterns, and locate the project outfalls. 20
Locate drainage constraints such as high and low points on the project profile, sites 21 that may be available for stormwater quality treatment or flow control, and the 22 locations of steep grades or cross-slopes, etc. 23
Prepare a drainage map showing the basins, sub-basins, cross drains, soil types, 24 project layout, various ground cover conditions, elevation contours, main flow paths, 25 outfalls, sensitive areas, and right-of-ways. 26
STEP 3 – Perform an Off-Site Analysis 27
An off-site analysis is required for all projects. The objective of the off-site analysis is to 28 evaluate and identify: 29
Conveyance system capacity problems, both cross-drains and on-site collection 30 systems 31
Localized flooding 32
Upland erosion impacts, including landslide hazards 33
Stream channel erosion at the outfall location 34
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Existing downstream flow conditions, for use when reviewing project impacts 1
STEP 4 – Prepare a Permanent Stormwater Management Plan 2
Select stormwater control Best Management Practices (BMPs) and facilities that will serve 3 the project site in its developed condition. This selection process is presented in detail in 4 Chapter 6 of this Guam Transportation Stormwater Drainage Manual (TSDM). The 5 permanent stormwater management system should contain the following main items: 6
Permanent Stormwater Management Plan – Existing Site Hydrology 7
If flow control facilities are proposed to comply with Minimum Requirement 6, provide a list 8 of assumptions and site parameters used in analyzing the existing site hydrology. The soil 9 types, existing streams and water bodies, existing drainage features, and land covers 10 affected by the project should be included on the drainage map. Additional exhibits showing 11 areas in more detail may be needed. The drainage map should also include: 12
Contour intervals with basin and sub-basin boundaries shown accurately 13
The delineation and acreage of basins and sub-basins 14
The conceptual water quality treatment and flow control facility locations 15
Outfalls 16
Overflow routes 17
The direction of flow and the project right-of-ways and limits should be indicated. Each 18 basin within or flowing through the site should be named for reference when performing the 19 hydrology calculations. 20
Permanent Stormwater Management Plan: Developed Site Hydrology 21
Totals of pre-development and post-development impervious and pervious surfaces must 22 be tabulated for each drainage basin within the project limits. These are needed to confirm 23 the threshold limits for the application of treatment facilities (Minimum Requirement 5) and 24 flow control facilities (Minimum Requirement 6) for the project. 25
Provide narrative, mathematical, and graphic presentations of hydrology input parameters 26 selected for the pre-developed and post-developed site conditions, including basin 27 acreages, soil types, and land covers. The basin descriptions should be cross-referenced to 28 the naming convention shown on the drainage map and to any calculation sheets and 29 computer printouts. Pre-development and post-development basin flows should be 30 tabulated, and will apply to both on- and off-site basins that the project intersects or 31 otherwise impacts. 32
Any documents used to determine the developed site hydrology should be included. 33 Whenever possible, maintain the same basin name as used for the pre-developed site 34 hydrology. If the boundaries of a basin have been modified by the project, that modification 35
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should be clearly shown on a map and the name modified to indicate the change. Final 1 grade topographic maps should also be provided. 2
Permanent Stormwater Management Plan: Performance Standards and Goals 3
If quality treatment facilities are proposed, provide a tabulation of the water quality criteria 4 used. If flow control facilities are proposed, provide a confirmation of the flow control 5 standard being achieved. Provide full support or backup for the reasons why the BMPs 6 were selected, how they were sized, and any other constraints that were taken into account 7 during the design process. Document efforts to limit stormwater discharge by infiltrating as 8 much of the runoff as possible within the project limits. 9
Permanent Stormwater Management Plan: Flow Control System 10
Provide a drawing of the flow control facility and its appurtenances. This drawing must show 11 the basic measurements necessary to calculate the storage volumes available from zero to 12 the maximum head, all orifice/restrictor sizes and head relationships, control 13 structure/restrictor placement, and placement on the site. 14
Include all final computer modeling printouts, calculations, equations, references, 15 storage/volume tables, and graphs, as necessary, to show the results and methodology 16 used to determine the storage facility volumes. 17
Permanent Stormwater Management Plan: Stormwater Quality Treatment System 18
Provide a drawing of the proposed stormwater quality treatment BMPs. The drawing must 19 show the overall measurements and dimensions, placement on the site, and 20 locations/details of inflow, bypass, and discharge systems. 21
Include any computer modeling printouts, calculations, equations, references, and graphs, 22 as necessary, to show that the facilities are designed to be consistent with the TSDM 23 requirements. 24
Permanent Stormwater Management Plan: Conveyance System Analysis and Design 25
Present an analysis of any existing conveyance systems and the analysis and design of the 26 proposed stormwater conveyance system for the project. This information should be 27 presented in a clear, concise manner that can be easily followed, checked, and verified. All 28 pipes, culverts, catch basins, channels, swales, and other stormwater conveyance 29 appurtenances must be clearly labeled and correspond directly to the engineering plans. 30 More complicated storm drain systems should be sized by routing the flow hydrographs 31 using a hydraulic modeling program. 32
Main cross-drains should be identified by name on the drainage map and all other 33 calculations and exhibits cross-referenced to that name. 34
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STEP 5 – Prepare Construction TESC Plan 1
See Appendix I for the development of the Temporary Erosion and Sedimentation Control 2 (TESC) plans, which are incorporated into the construction SWPPP. 3
STEP 6 – Complete the Hydraulic Report 4
This document encompasses the entire drainage management plan, which is reviewed by 5 DPW prior to the start of construction. See Appendix II for the hydraulic report outline and 6 additional preparation guidelines. 7
STEP 7 – Check Compliance with All Applicable Minimum Requirements 8
A correctly designed and implemented permanent stormwater management plan should 9 specifically fulfill all minimum requirements applicable to the project. The plan should be 10 reviewed to check that these requirements are satisfied. 11
3.2.2 MINIMUM REQUIREMENT 2: CONSTRUCTION STORMWATER 12 POLLUTION PREVENTION 13
All projects with a disturbance of 1 acre or more, or smaller projects that are part of a larger 14 program plan greater than 1 acre, are required to prepare and implement a SWPPP in 15 accordance with the National Pollutant Discharge Elimination System (NPDES) Phase II 16 Stormwater Program. The USEPA Web site is located at http://cfpub.epa.gov/npdes/ 17 stormwater/swphases.cfm. The NPDES Notice of Intent application page is located at 18 http://cfpub.epa.gov/npdes/stormwater/application_coverage.cfm. In addition, a SWPPP will 19 be required where the project impacts a Guam Environmental Protection Agency (GEPA) 20 designated stormwater “hotspot”. The EPA’s construction stormwater permit and SWPPP 21 template can also be found at http://cfpub.epa.gov/npdes/stormwater/const.cfm. The 22 stormwater hotspot is defined as a land use or activity that generates higher concentrations 23 of hydrocarbons, trace metals, or toxins than are found in typical stormwater runoff, based 24 on monitoring studies (See Section 2 of the Commonwealth of Northern Mariana Islands 25 (CNMI) and the Guam Stormwater Management Manual, Vol. 1: Final – October 2006, for 26 more details on GEPA-designated hotspots). The two main components of the SWPPP are: 27
Temporary Erosion and Sedimentation Control (TESC) 28
Spill Prevention, Control, and Countermeasures (SPCC) planning 29
Together, TESC and SPCC plans should satisfy the construction SWPPP requirements. 30 The objective of SWPPP preparation is to ensure that construction projects do not impair 31 water quality by allowing sediment discharge from the site or allowing pollutant spills. 32
Further to the project areas requiring a SWPPP (as listed above), a TESC plan and report 33 must be prepared for all DPW projects that are not otherwise exempt in accordance with 34 Section 3.1.1. An initial TESC design is usually included in the bid package. After project 35 award and before construction, the contractor will be required to accept or update the bid 36
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document’s TESC plan. The TESC plan may need to be modified to fit the contractor’s 1 staging and work plan elements. 2
DPW exempt projects must still fully-address construction erosion control and the 12 3 elements of TESC. However, a stand-alone TESC report and plan is not required. 4
See Appendix I for details on each element of TESC (See Chapter 7 for design and 5 applicability guidelines for TESC BMPs). Below are the 12 TESC elements: 6
ELEMENT 1 - Mark clearing limits 7
ELEMENT 2 - Establish construction access 8
ELEMENT 3 - Control flow rates 9
ELEMENT 4 - Install sediment controls 10
ELEMENT 5 - Stabilize soils 11
ELEMENT 6 - Protect slopes 12
ELEMENT 7 - Protect drain inlets 13
ELEMENT 8 - Stabilize channels and outlets 14
ELEMENT 9 - Control pollutants 15
ELEMENT 10- Control de-watering 16
ELEMENT 11- Maintain BMPs 17
ELEMENT 12- Manage the project 18
All construction site measures should be designed to accommodate (i.e., safely convey 19 without creating erosive conditions) the 10-year frequency storm. All temporary sediment 20 trapping devices should be designed to treat runoff from a minimum of the three-year 21 frequency storm. 22
All projects involving mechanized equipment or construction material that could potentially 23 contaminate stormwater or soils require SPCC plans. The SPCC plan is another element of 24 the contractor’s SWPPP, (prepared by the contractor) and contains the following: 25
Site information and project description 26
Spill prevention and containment 27
Spill response 28
Material and equipment requirements 29
Reporting information 30
Program management 31
Plans to contain preexisting contamination, if necessary 32
3 - 8 August 2010
3.2.3 MINIMUM REQUIREMENT 3: SOURCE CONTROL OF POLLUTANTS 1
All known, available, and reasonable source control BMPs must be applied and must be 2 selected, designed, and maintained in accordance with the TSDM. The primary objective of 3 this requirement is prevention. It is more cost-effective to prevent pollutants from coming in 4 contact and mixing with the stormwater rather than removing pollutants after they have 5 mixed with runoff. 6
Source control BMPs include operational and structural BMPs. Operational BMPs are 7 practices that prevent (or reduce) pollutants from entering stormwater. Examples include 8 preventive maintenance procedures (such as sweeping pavements, cleaning of sediment 9 traps and catch basins, etc.), spill prevention and cleanup, and the inspection of potential 10 pollutant sources. 11
Structural BMPs are physical, structural, or mechanical devices or facilities intended to 12 prevent pollutants from entering stormwater. Examples include installing vegetation for 13 temporary and permanent erosion control, putting roofs over outside storage areas, and 14 putting berms around potential pollutant source areas to prevent both stormwater run-on 15 and pollutant runoff. See Chapter 7 for criteria on the design of source control BMPs. 16
3.2.4 MINIMUM REQUIREMENT 4: MAINTAINING THE NATURAL DRAINAGE 17 SYSTEM 18
The objective of this minimum requirement is to preserve and use natural drainage systems 19 to the fullest extent because of the multiple stormwater benefits these systems provide, and 20 to prevent erosion at and downstream of the discharge location. 21
Discharges from the project site should occur at the natural location, to the maximum extent 22 practicable. The runoff must avoid adverse impacts to downstream receiving waters and 23 down-gradient properties. All outfalls must be reviewed for erosion potential and energy 24 dissipation applied as necessary. The designer must use the best available design 25 practices to maintain hydrologic function and drainage patterns based on site geology, 26 hydrology, and topography. 27
Project area stormwater should infiltrate and/or discharge into the same watershed per 28 watershed boundary encompassing that portion of the project. Moving of project area runoff 29 from one watershed to another is not allowable. 30
It is desirable and usually most cost effective to keep on-site stormwater separated from off-31 site flows prior to the project area runoff treatment and discharge (or infiltration). 32 Stormwater management plans should attempt to maintain off-site flows in the existing or 33 natural waterway with the least disturbance or change in hydraulic conditions possible. 34
See also Section 3.2.6, Minimum Requirement 6: Flow Control, for additional criteria and 35 details. 36
3 - 9 August 2010
3.2.5 MINIMUM REQUIREMENT 5: RUNOFF TREATMENT 1
The primary objective of runoff treatment is to reduce pollutant loads and concentrations in 2 stormwater runoff using physical, biological, and chemical removal mechanisms to maintain 3 or enhance the beneficial uses of receiving waters. 4
Additional requirements include: 5
Infiltration is considered a treatment for northern Guam; however, all stormwater 6 infiltrated should be performed in accordance with the appropriate infiltration BMP 7 requirements listed in Chapter 6. 8
Refer to Chapter 2 for incorporating the stormwater treatment strategy into the 9 project development process. 10
Select the appropriate BMPs in accordance with the guidelines in Chapter 6. The 11 BMPs should be sized in accordance with the hydrology and hydraulics methods 12 listed in Chapters 4 and 5. 13
3.2.5.1 RUNOFF TREATMENT EXCEPTIONS 14
Runoff treatment is not required on: 15
Projects where the only work involved is the addition of paved surfaces not intended 16 for use by motor vehicles, such as sidewalks or bike trails, and that are separate 17 from the adjacent roadway. 18
Maintenance-related activities such as fixing of potholes. 19
Exemptions upon request by demonstrating and documenting a physical limitation 20 that would make implementation impractical. 21
3.2.5.2 CRITERIA 22
The stormwater quality treatment volume for new development (i.e., new transportation 23 facility where no roadway presently exists) should meet the following criteria: 24
All project area stormwater should be contained on site and treated prior to 25 discharge and/or infiltration. Infiltration is a preferred BMP. The designer should 26 make all efforts possible to use infiltration over discharge. 27
The stormwater quality treatment volume should meet the 90% Rule, when 28 discharge is to high quality waters and/or involving hotspot land uses. The 29 stormwater quality volume should meet the 80% Rule where project discharge is to 30 moderate quality waters. High quality waters, hotspot land uses, and moderate 31 quality waters are defined in Volume I, Chapter 2 of the CNMI and Guam 32 Stormwater Management Manual, Vol. 1: Final – October 2006. See Figure 3-1 for a 33 delineation of the high quality and moderate quality water body areas. 34
3 - 10 August 2010
The stormwater quality treatment volume for redevelopment (i.e., any transportation project 1 involving improvements to an existing facility or roadway) should meet the following criteria: 2
All project area stormwater should be contained on site and the appropriate 3 amounts of pavement area runoff treated prior to discharge and/or infiltration. 4
Provide stormwater treatment BMPs where they do not currently exist for at least 40 5 percent of the impervious cover. Applicability of the 90% or 80% Rules should be 6 governed by the discharge location. 7
The 80% and 90% Rules: The stormwater quality volume is the amount of stormwater to 8 be treated in accordance with the runoff from a storm event that corresponds to the 90 or 9 80 percentile amount of total annual average rainfall. In accordance with the statistical 10 analysis of the rainfall for Guam. The 80 percent amount corresponds to 56 percent of the 11 two-year, one-hour storm precipitation depth, whereas the 90 percent amount corresponds 12 to 72 percent of the two-year, one-hour storm precipitation depth. Flow-based quality 13 treatment BMPs should be designed for the peak runoff flow rate from the two-year, one-14 hour storm event. 15
See Chapters 4 and 5 for methods to calculate these runoff volumes and corresponding 16 flow rates. See Chapter 6 for additional guidelines on the applicability and criteria for the 17 associated treatment BMPs. 18
As much of the project area stormwater as practical should be infiltrated, in particular within 19 the limestone bedrock regions of Guam. Pre-treatment of stormwater is required before 20 infiltration. The designer should also investigate the possibility of using soil amendments to 21 enhance the suitability of sites for infiltration for GEPA-defined hotspot locations. One 22 hundred percent of the project area stormwater requires quality treatment before infiltration. 23 See the applicable infiltration BMP descriptions in Chapter 6 for more details. 24
3 - 11 August 2010
1 Figure 3-1: Water Quality Classification – Water Bodies of Guam 2
3
3 - 12 August 2010
3.2.6 MINIMUM REQUIREMENT 6: FLOW CONTROL 1
Projects must provide flow control to reduce the impacts of stormwater runoff from 2 impervious surfaces and land cover conversions. This requirement applies to all nonexempt 3 projects that discharge stormwater directly or indirectly through a conveyance system to a 4 surface freshwater body. The objective of flow control is to prevent increases in the stream 5 channel erosion rates beyond those characteristics of natural existing conditions. 6
3.2.6.1 CRITERIA 7
Flow control requires 24 hours of detention of the post-developed, one-year, one-hour 8 storm event runoff from the project area. The one-year, one-hour storm event runoff is 9 captured and released over a 24-hour period. Also, the post-development project discharge 10 rates must not exceed pre-development peak discharge rates for the 25-year, one-hour 11 storm event. Detention volumes for the 25-year, one-hour storm event should be emptied 12 within a 72-hour period from the start of the design storm event. 13
Depending on the shape and land use of a watershed, it is possible that upstream peak 14 discharge may arrive at the same time as the project is releasing its peak discharge (even 15 with detention), thus increasing the total downstream discharge. A downstream analysis is 16 thus required for projects over 50 acres with on-site impervious cover greater than 25 17 percent or when deemed appropriate by the reviewing authority when existing conditions 18 are already causing a problem (such as known flooding or channel erosion problems). The 19 downstream analysis is a hydrologic and hydraulic evaluation extended downstream to the 20 point where the site represents 10 percent of the total drainage area. If the flow rates and 21 velocities increase by more than 5 percent, then the project detention must be redesigned 22 to lessen the impacts. 23
It is desirable and usually more cost effective to infiltrate project area stormwater to the 24 maximum extent possible, particularly in the limestone bedrock area of northern Guam. All 25 project stormwater must be contained and conveyed on site for detention (prior to discharge 26 to a natural water body) or infiltration within the project limits. Project stormwater facilities 27 should be checked for the 100-year event, and provided with a suitable over-topping type 28 discharge point to a natural water body without damage or flooding of neighboring 29 properties. Where project stormwater discharge is by infiltration alone, without a suitable 30 over-topping discharge site, then the 100-year storm runoff volume should be contained. 31
See Figure 6-2 in Chapter 6 for the selection of flow control BMPs, in the event flow control 32 is required. 33
3.2.6.2 EXEMPTIONS 34
Projects able to disperse stormwater without discharging runoff either directly or 35 indirectly through a conveyance system to surface waters, such as infiltration and 36 dispersion BMPs. 37
Projects discharging runoff to coastal waters subject to tidal action. 38
3 - 13 August 2010
Project areas from over-water structures like bridges, docks, and piers (the portion 1 of an over-water structure over the ordinary high-water mark). 2
Projects that discharge through man-made conveyance directly to a large reservoir 3 or lake, or a stream or river with a contributory drainage area greater than 5 square 4 miles. 5
Exemptions upon request by demonstrating and documenting a physical limitation 6 that would make implementation impractical. 7
3.2.7 MINIMUM REQUIREMENT 7: WETLANDS PROTECTION 8
This requirement is in place to ensure that wetlands receive the same level of protection as 9 any other waters of Guam. Wetlands are extremely important natural resources that provide 10 multiple stormwater benefits, including groundwater recharge, flood control, and stream 11 channel erosion protection. They are easily impacted by development unless careful 12 planning and management are conducted. 13
Wetlands can be severely degraded by stormwater discharges from urban development 14 due to pollutants in the runoff, and also due to disruptions of the natural hydrologic 15 functioning of the wetlands system. Changes in water levels and the frequency and duration 16 of inundations are of particular concern and should not be altered by the project. 17
Wetlands and streams should be protected by limiting clearing within 25 ft. from top of 18 bank, with perimeter sediment controls during construction. Stormwater discharges should 19 be performed using low impact type spreader trenches or swales for a more natural non-20 erosive sheet flow into the water body. 21
These requirements apply to projects where stormwater discharges into a wetland, either 22 directly or indirectly, through a conveyance system. All project area discharge to a wetland 23 must meet the requirements of Section 3.2 5 for quality treatment and Section 3.2.6 for flow 24 control. 25
3.2.8 MINIMUM REQUIREMENT 8: INCORPORATING WATERSHED 26 PLANNING INTO STORMWATER MANAGEMENT 27
This requirement is meant to promote watershed-based planning as a means of developing 28 and implementing comprehensive water quality protection measures. The primary 29 objectives of watershed-based planning are to reduce pollutant loads and hydrologic 30 impacts to surface water and groundwater in order to protect beneficial uses. 31
Although Minimum Requirements 1 through 7 establish general standards for individual 32 sites, they do not evaluate the overall pollution impacts and protection opportunities that 33 could exist at the watershed level. In order for a watershed plan to serve as a means of 34 modifying the Minimum Requirements, the following conditions must be met: 35
The plan must be formally adopted by all jurisdictions with responsibilities under the 36 plan 37
3 - 14 August 2010
All ordinances or regulations called for by the plan must be in effect 1
3.2.9 MINIMUM REQUIREMENT 9: OPERATIONS AND MAINTENANCE 2
The objective of this requirement is to ensure that stormwater control facilities can be 3 adequately maintained and operated properly. This will better ensure that preventive 4 maintenance and performance inspections are completed regularly. 5
Chapter 6 describes some of the general maintenance requirements that are identified with 6 specific BMPs. In addition, the designer should review the stormwater management 7 concepts with DPW’s maintenance staff for adequacy of access to allow for regular and 8 convenient maintenance and inspection of facilities. This review will further improve the 9 designer’s ability with the BMP configurations, where specific design parameters are based 10 on maintainability factors. 11
12
4 - 1 August 2010
4 HYDROLOGIC ANALYSIS 1
4.1 GENERAL HYDROLOGY 2
Hydrology is often defined as the science that addresses the physical properties, 3
occurrences, and movements of water in the atmosphere, on the surface of the Earth, and 4
in the outer crust of the Earth. For the Highway Designer, the primary focus of hydrology is 5
the water that moves on the Earth's surface and, in particular, that part that ultimately 6
crosses transportation corridors (i.e., highway stream crossings). A secondary interest is 7
providing interior drainage for roadways, median areas, and interchanges. 8
The runoff of water over land has long been studied, and some rather sophisticated 9
theories and methods have been proposed and developed for estimating flood flows. Most 10
attempts to describe the process have been only partially successful at best. This is due to 11
the complexity of the process and interactive factors. Although hydrology is not an exact 12
science, it is possible to obtain solutions that are functionally acceptable to form the basis 13
for the design of roadway drainage facilities. 14
Regardless of the size or cost of the drainage feature, the most important step prior to 15
hydraulic design is estimating the discharge (rate of runoff) or volume of runoff that the 16
drainage facility will be required to convey or control. The extent of such studies should be 17
commensurate with the importance of the highway, potential for damage to the highway, 18
loss of property, and hazard to life associated with the facilities. The choice of analytical 19
method must be a conscious decision made as each problem arises. To make an informed 20
decision, the Highway Engineer must determine: 21
What level of hydrologic analysis is justified? 22
What data are available or must be collected? 23
What methods of analysis are available, including the relative strengths and 24
weaknesses in terms of costs and accuracy? 25
For example, cross drainage design normally requires more extensive hydrologic analyses 26
than what is necessary for roadway drainage design. The well-known and relatively simple 27
Rational Method is generally adequate for estimating the rate of runoff for the design of on-28
site roadway drainage facilities and the removal of runoff from highway pavement. 29
Alternately, more complex hydrograph or modeling methods are usually required for 30
estimating flows from larger off-site watersheds. 31
4.1.1 GENERAL PROCEDURES 32
Drainage studies often follow a similar sequence of calculations for all procedures because 33
precipitation must be routed through watersheds, channels, and reservoirs. In most cases, 34
stormwater runoff will be estimated using the following general procedure: 35
4 - 2 August 2010
Divide the watershed into appropriate sub-areas to correspond with homogenous 1
land use conditions and the placement of drainage facilities such as inlets, ponds, 2
and open channels. 3
Collect and analyze watershed data. Every drainage problem is unique, so the need 4
for a field survey that appraises and collects site-specific hydrologic and hydraulic 5
data cannot be overstated. Any historical and maintenance records should also be 6
collected. 7
Establish design storm characteristics (frequency, rainfall depth, and intensity) as 8
appropriate for the structural element and design procedure selected. 9
Calculate the peak runoff rate or determine the time distribution of rainfall excess. 10
No further calculations are generally required if only the peak runoff rate is desired. 11
Develop a Unit Hydrograph (UH) for the watershed if a runoff hydrograph is desired 12
and the procedure selected uses a UH. 13
Develop the direct runoff hydrograph or rainfall excess determined above, as 14
appropriate. 15
Perform downstream channel and reservoir routing as appropriate. 16
4.1.2 PROCEDURE SELECTION 17
Stream flow measurements for determining peak runoff rates for pre-project conditions are 18
usually unavailable. In such cases, it is accepted practice to estimate peak runoff using 19
several different methods. In general, results should be compared, not averaged, and the 20
method that best reflects project conditions should be used, with the reasons well 21
documented. 22
A consideration for peak runoff rates for design conditions is generally adequate for 23
conveyance systems such as storm drains or open channels. However, if the design must 24
include flood routing (e.g., storage basins or complex conveyances), a flood hydrograph is 25
usually required. Although the development of a runoff hydrograph is often accomplished 26
using computer programs, only desktop procedures are presented in this Transportation 27
Stormwater Drainage Manual (TSDM). 28
Applicable methods for determining peak runoff rates and flood hydrographs for various 29
size drainage basins are presented in Table 4-1. 30
Applicable Runoff Determination Methods 31
Rational Method: The Rational Method provides peak runoff rates for small and 32
intermediate urban or rural watersheds, but it is best suited for urban storm systems. 33
It should be used with caution if the Time of Concentration (Tc) exceeds 30 minutes. 34
Where the Tc exceeds 30 minutes, the basin should be broken down by topographic 35
and/or land cover conditions and the separate flows routed through the conveyance 36
to the point of interest. 37
4 - 3 August 2010
Guidelines for Selecting Peak Runoff Rate and Flood Hydrograph Methods
Peak Runoff Rates Flood Hydrographs N
o
Wa
ters
he
d
cate
go
ry
Str
ea
m F
low
An
aly
sis
Ra
tio
na
l M
eth
od
Na
tura
l F
low
Re
gre
ss
ion
Eq
ua
tio
ns
SC
S S
yn
theti
c
Un
it H
yd
rog
rap
h
Me
tho
ds a
nd
CN
Me
tho
d
SC
S T
R-5
5
Me
tho
d
Un
it H
yd
rog
rap
h
Th
eo
ry
Sa
nta
Barb
ara
Urb
an
Hy
dro
gra
ph
Me
tho
d
SC
S T
R-5
5
Me
tho
d
1 Small Urban 0-200 acres X X X X X X X
2 Large Urban + 200 acres X X1A X X
3 Small Rural 0-200 acres X X X X X X X
4 Medium Rural 0-1,000 acres X X1A X X X X
5 Large Rural 1,000 acres-6.5 square miles X X X X
6 Major + 6.5 square miles X X X
1A: Can be utilized for areas larger than 200 acres if the watershed is broken down into smaller areas and each area’s runoff is routed through the conveyance reach.
Table 4-1: Guidelines for Selecting Peak Runoff Rate and Flood Hydrograph Methods
4 - 4 August 2010
Regression Equations: Regression equations are alternative runoff calculation 1
methods for natural flow conditions. Regression equations are developed for 2
specific watersheds having recorded peak flow data by using statistical analytical 3
techniques. These equations (with appropriate adjustments) can then be used for 4
adjacent watersheds with similar characteristics. 5
Curve Number and Unit Hydrograph Methods: The USDA, National Resources 6
Conservation Service (NRCS) runoff equation, Curve Number (CN), and UH 7
procedures can be adapted to a variety of conditions for both rural and urban 8
watersheds. The NRCS combined CN method and runoff equation is widely used in 9
estimating direct runoff volume because of its simplicity, flexibility, and availability. 10
The only input parameter needed is CN, and the hydrologic data used to estimate 11
CN are normally available for most un-gauged watersheds. Because the CN is the 12
only parameter required, the runoff prediction is entirely dependent on the accuracy 13
of the CN. A runoff hydrograph and the peak discharge rate can be calculated 14
directly from the runoff equation using the NRCS UH procedures. 15
TR-55: The NRCS Technical Release TR-55 (TR-55) is a good reference with 16
regards to application of the CN runoff equation. However, the graphical and tabular 17
methods presented in the technical release are not applicable to the Guam 18
transportation projects, since they are based on the NRCS standard rainfall 19
distribution 24-hour storms. See Section 4.2 for the applicable Guam design storms. 20
Note that the CN runoff equation is usually restricted to estimating peak runoff 21
volume for homogenous areas less than 300 acres. 22
Santa Barbara Urban Hydrograph: The Santa Barbara Urban Hydrograph 23
(SBUH) method will develop a synthetic runoff hydrograph, applicable to small and 24
large urban watersheds, as long as an appropriate delineation of homogenous land 25
use areas is performed. The SBUH was developed by the Santa Barbara Flood 26
Control and Water Conservation District in California as a short cut to the NRCS 27
UH. The primary difference between the SBUH and the NRCS UH method is the 28
use of the unit hydrograph in computing the runoff hydrograph. The SBUH replaces 29
the unit hydrograph with an algorithm. The SBUH method greatly simplifies the 30
computation of runoff hydrographs and is particularly applicable for computer 31
modeling. 32
Hydrology Modeling Programs: The use of computer programs, which usually 33
allow both hydrology and hydraulic analyses, is highly recommended for the routing 34
of complex systems. There are a number of programs available, including public 35
domain programs such as WinTR20, that use the NRCS CN and UH methods for 36
developing the flow rates and volumes. Most hydrology/hydraulic modeling 37
programs will also allow the use of the SBUH method for developing the runoff 38
hydrographs. 39
40
4 - 5 August 2010
4.1.3 PRECIPITATION CHARACTERISTICS IN DRAINAGE CALCULATIONS 1
The characteristics of precipitation typically considered in drainage calculations include: 2
Intensity 3
Duration 4
Time distribution of rainfall (hyetograph) 5
Storm shape, size, and movement 6
Frequency 7
Intensity: Intensity is the rate of precipitation, commonly given in units of inches per hour. 8
In any given storm, the instantaneous intensity is the slope of mass rainfall curve at a 9
particular time. From the standpoint of drainage calculations, intensity is perhaps the most 10
important of the rainfall characteristics because discharge from a given watershed will 11
increase as the intensity rises. See Figure 4-1 for the intensity curves to use on Guam 12
Transportation Projects. 13
Duration: Duration is the time length of the precipitation event. The precipitation durations 14
used in Figures 4-1 and 4-2 have been selected based on statistical analysis of recorded 15
precipitation events (see Appendix III). The Department of Public Works (DPW) is using a 16
one-hour storm event as the standard storm duration for design and analysis purposes of a 17
surface water management system for transportation projects. 18
Time Distribution of Rainfall or Hyetograph: A hyetograph provides a distribution of 19
incremental precipitation versus time. In practice, depth duration data for particular design 20
frequency are generally used to develop a synthetic design storm event. A design storm 21
distribution with a one-hour duration will be used for transportation projects on Guam (see 22
Figure 4-2). 23
Shape, Size, and Movement: Storm shape, size, and movement are normally determined 24
by the type of storm. All three factors determine the areal extent of precipitation and the 25
size of the drainage area that contribute over time to the surface runoff. 26
Rainfall varies spatially on the island owing to orographic effects (increases in rainfall with 27
altitude). Mean annual rainfall is less than 90 in. on some coastal lowland areas and greater 28
than 115 in. on mountainous areas in southern Guam. Since temperature and humidity are 29
fairly uniform throughout the year, the variations of wind and rainfall are what define the 30
seasons in Guam. 31
Highly seasonal rainfall and wind patterns in the region provide Guam with distinctive wet 32
and dry seasons. The dry season (January through May) is dominated by northeasterly 33
trade winds with scattered and light showers. Rainfall during the dry season accounts for 15 34
to 20 percent of the total annual rainfall. The wet season (July through November) accounts 35
for an average of 65 percent of the annual rainfall. During the rainy season, westerly-36
moving storm systems and occasional typhoons bring heavy, steady rain and strong winds. 37
4 - 6 August 2010
Monthly rainfall totals vary from less than 1 in. from February to April to more than 20 in. 1
from August to November. Exceptionally dry years recur about once every four years in 2
correlation with episodes of El Nino Southern Oscillation in the Pacific. 3
Frequency: Frequency quantifies the likelihood of the recurrence of precipitation with a 4
given duration and average intensity. The major concern of highway design is with the 5
frequency of occurrence for the surface runoff resulting from precipitation and, in particular, 6
the frequency of the peak discharges. See Chapter 5, Section 5.2 for a more detailed 7
description of the frequency policy for Guam transportation projects. 8
Refer to Chapter 2 of the FHWA publication Hydraulic Design Series-2, Highway Hydrology 9
(HDS-2) for additional information on precipitation characteristics. 10
4.1.4 RUNOFF COMPONENTS 11
The four main components of runoff generally considered in stream flow data analysis are: 12
Overland flow 13
Channel precipitation 14
Interflow 15
Groundwater flow 16
Overland flow travels over the ground surface to the stream channel. Channel precipitation 17
is direct rainfall on the water surface. Runoff that enters a stream channel by traveling 18
laterally through the upper layer of soil is called interflow, while stream flow generated by 19
the occurrence of water table conditions above the channel bottom is called groundwater or 20
base flow. 21
This chapter addresses the relationships between precipitation and overland flow. 22
Precipitation that does not convert to overland flow is lost to evaporation, soil storage, and 23
infiltration. The infiltrated portion becomes interflow and groundwater flow. 24
Refer to Chapter 2 of the FHWA publication HDS-2 for detailed information on runoff 25
characteristics. 26
4.2 DESIGN STORM SELECTION 27
Stormwater management facilities must be sized to create a logical balance between the 28
safety of the project and the economics of installation, maintenance, and replacement. 29
Accordingly, different aspects of stormwater conveyance, quality treatment, and flow control 30
are designed to different levels or frequencies of storm events. These design storm events 31
are selected based on an updated statistical analysis of the Guam precipitation records. 32
The details of the statistical analysis and discussion on the selection of the design storms 33
are discussed in Appendix III. 34
A one-hour duration storm will be used for the design of transportation stormwater facilities 35
for Guam. The one-hour storm duration was selected from statistical analysis that showed 36
4 - 7 August 2010
that most precipitation on Guam occurs during intense, short duration storms. The 1
probability of getting storms longer than a one-hour duration, as well as having more than 2
one one-hour storm in a day, is statistically remote. 3
The one-hour storm distribution is a hybrid curve based on an analysis of the 15-minute 4
rainfall records using a partial duration statistical series, and the lesser 30- and 60-minute 5
precipitation amounts determined from the one-hour storm statistical series. The 6
subsequent intensities for durations shorter than 15 minutes are somewhat conservative, 7
while the longer duration intensities closely match the statistically-predicted rainfall depths. 8
Stormwater management systems that are developed using the selected design storm 9
events and intensity curves will size the drainage facilities to match actual rainfall 10
conditions. This will help in preparing an efficient design that provides for the public safety, 11
while minimizing project costs. 12
One-hour precipitation values and intensity-duration curves for the design storm 13
frequencies are shown in Figure 4-1. These precipitation intensities should be used for 14
design of the transportation stormwater management facilities. Figure 4-2 is the standard 15
one-hour storm distribution to use for design purposes. The standard one-hour storm 16
distribution is also shown in a tabular format in Table 4-2. Figure 4-3 shows the precipitation 17
depths in inches for different statistical recurrence intervals for the one-hour duration storm. 18
19
Figure 4-1: Intensity-Duration-Frequency (IDF) Curves for a One-Hour Duration Storm 20
4 - 8 August 2010
1
Figure 4-2: Design Rainfall Distribution for a One-Hour Duration Storm 2
3
4
5
6
Figure 4-3: Rainfall (inches) per Recurrence Period for a One-Hour Duration Storm 7
8
4 - 9 August 2010
Time (Minutes) Rainfall Fraction
5 0.315
10 0.435
15 0.525
20 0.600
25 0.666
30 0.725
35 0.778
40 0.828
45 0.875
50 0.919
55 0.960
60 1.000
Table 4-2: One-Hour Storm Distribution 1
4.3 TIME OF CONCENTRATION 2
The Tc is the time required for a hydraulic wave to travel across a watershed. It is often 3
approximated as the time required for runoff to travel from the most hydraulically remote 4
part of the watershed to the point of interest. Most peak discharge, hydrograph, and 5
channel routing calculation methods use computed travel times. Thus, estimating travel 6
times are central to a variety of hydrologic design problems. 7
A segmental approach for calculating the Tc, commonly known as the velocity method, is 8
recommended for most applications. The velocity method of estimating the Tc typically 9
requires an evaluation of three flow components: 10
Overland flow 11
Rill, shallow channel, and street gutter flow 12
Open channel flow 13
Overland flow is sheet flow over plane surfaces, usually limited to a maximum length of 14
300 ft. After 300 ft., overland flow generally becomes concentrated into rills, small channels, 15
or gutters. Open channel flow is appropriate when the main conveyance system is 16
encountered. 17
After short distances, sheet flow tends to concentrate in rills and then gullies of increasing 18
proportions. Such flow is usually referred to as shallow concentrated flow. The flow in 19
gullies is usually collected by channels or pipes. 20
The travel time for any flow segment can be estimated as the ratio of flow length to the 21
velocity of flow, as follows: 22
23
4 - 10 August 2010
1
2
Equation 4-1 3
Where: 4
ti = Travel time for velocity segment i in min. 5
Vi = Average velocity for segment i in ft./s 6
Li = Length of the flow path for segment i in ft. 7
The number of flow segments or flow components considered should best represent the 8
actual flow path of the watershed being evaluated. The relationship for calculating the time 9
of concentration is expressed as: 10
Tc=t1+t2+t3+…tn 11
Equation 4-2 12
Where: 13
Tc = Watershed time of concentration in min. 14
t1 = Overland flow travel time in min. 15
t2 = Rill, shallow channel, street gutter flow travel time in min. 16
t3 = Open channel flow travel time in min. 17
tn = Travel time for segment i in min. 18
If the calculated Tc is less than five minutes, a minimum of five minutes should be used as 19
the duration. Procedures for estimating the average flow velocities and travel times are 20
discussed in Section 2.6 of the FHWA publication HDS-2. 21
4.4 PROCEDURES FOR DETERMING PEAK RUNOFF FLOW 22
Peak discharge is the maximum rate of the flow of water passing a given point during or 23
after a rainfall event. Peak runoff rates are automatically obtained when a flood hydrograph 24
is developed. However, in some situations, a peak runoff rate can be obtained without 25
developing a complete flood hydrograph through the use of the Rational Method, 26
regression equations, NRCS curve number equations, and UH relationships. 27
Design discharges, expressed as the quantity (Q) of flow in cubic feet per second are the 28
peak discharges that a roadway drainage structure is sized to handle. Some of the more 29
commonly used empirical methods for estimating runoff are as follows: 30
4.4.1 RATIONAL METHOD 31
Undoubtedly, the most popular and most often misused empirical hydrology method is the 32
rational formula: 33
4 - 11 August 2010
Q = C I A 1
Where: 2
Q = Design discharge in ft.³/s 3
C = Coefficient of runoff; function of watershed cover, and flood frequency 4
I = Rainfall intensity in in./hr 5
A = Drainage area in acres 6
Assumptions 7
The assumptions in the rational formula are as follows: 8
The drainage area should be smaller than 200 acres. 9
Peak discharge occurs when the entire watershed is contributing. 10
A storm that has a duration equal to Tc produces the highest peak discharge for this 11
frequency. 12
The rainfall intensity is uniform over a storm time duration equal to the Tc. The Tc is 13
the time required for water to travel from the most hydrologically remote point of the 14
basin to the outlet or point of interest. 15
The frequency of the computed peak flow is equal to the frequency of the rainfall 16
intensity. In other words, the 10-year rainfall intensity, i, is assumed to produce the 17
10-year peak discharge. 18
4.4.1.1 RUNOFF COEFFICIENTS 19
The runoff coefficient, C, is a function of ground cover. It represents the percent of water 20
that will run off the ground surface during the storm. Commonly accepted runoff coefficients 21
are discussed in Table 5.7 of FHWA publication HDS-2. 22
Should the basin contain varying amounts of different covers, a weighted runoff coefficient 23
for the entire basin can be determined as: 24
25
26
27
Where: 28
Ci = Runoff coefficient for cover type i that covers area Ai 29
A = Total area 30
31
32
4 - 12 August 2010
4.4.1.2 RAINFALL INTENSITY 1
After the appropriate storm frequency for the design has been determined (see Chapter 5) 2
and the time of concentration has been calculated, the rainfall intensity can be calculated. 3
Designers should never use a time of concentration that is less than five minutes for 4
intensity calculations, even when the calculated time of concentration is less than five 5
minutes. The five-minute limit is based on two ideas: 6
1. Shorter times give unrealistic intensities. The Intensity-Duration-Frequency (IDF) 7
curves are constructed from curve-smoothing equations and not based on actual 8
data collected at intervals shorter than 15 minutes. Making the curves shorter 9
involves extrapolation, which is not reliable. 10
2. It takes time for rainfall to generate into runoff within a defined basin; thus, it would 11
not be realistic to have less than five minutes for a time of concentration. 12
It should be noted that the rainfall intensity at any given time is the average of the most 13
intense period enveloped by the time of concentration and is not instantaneous rainfall. 14
Figure 4-1 contains the IDF relationships to be used for design, where the duration in the 15
graph matches the Tc (in minutes) calculated for the drainage basin. Table 4-3 lists the 16
equations of the curves shown in Figure 4-1, where x is the duration in minutes and y is the 17
rainfall intensity in inches per hour. Note that these equations are only applicable for 18
durations from 5 to 60 minutes. 19
RETURN PERIOD (Yrs) IDF Equation
100 y = 20.971x-0.476
50 y = 20.04x-0.492
25 y = 19.187x-0.512
10 y = 18.086x-0.545
3 y = 16.781x-0.613
2 y = 16.322x-0.651
1 y = 18.591x-1.067
Table 4-3: Rainfall Intensity Equations for a One-Hour Duration Storm 20
4.4.2 NRCS CURVE NUMBER METHOD 21
In the early 1950s, the USDA Soil Conservation Service (now called the National 22
Resources Conservation Service or NRCS) developed a method for estimating the volume 23
of direct runoff from storm rainfall and evaluating the effects of land use and treatment 24
changes on the volume of direct runoff. The method is often referred to as the CN method, 25
and it was empirically developed from small agricultural watersheds. However, the 26
procedure, if carefully utilized, is applicable to other areas. This CN method provides results 27
in terms of depth (convertible to volume by multiplying depth by the area) of runoff, but it is 28
4 - 13 August 2010
not applicable to the estimation of peak design discharge unless the design hydrograph is 1
developed. For a description of the hydrograph development method used by the NRCS, 2
see NRCS publication National Engineering Handbook, Section 4, Hydrology, dated 1985. 3
See also the NRCS publication Technical Paper TP-149, dated 1973. The routing of 4
hydrographs through a watershed is explained in NRCS Technical Release TR-20, dated 5
1973. TR-20 has now been updated as a public domain computer program which will 6
develop runoff hydrographs and route them through a watershed. 7
The use of TR-20 (WinTR20 computer program) is most applicable for determining flows 8
from larger basins, such as streams, where sizing of cross-culverts or bridge openings is 9
needed. The design one-hour storm precipitation (Figure 4-3) and storm rainfall distribution 10
(Table 4-2) can be applied directly in the program. 11
Simplified procedures for estimating runoff volumes from small urban watersheds is 12
presented in NRCS publication Technical Release TR-55, Urban Hydrology for Small 13
Watersheds, June 1986. This publication provides a description of the runoff CN method. 14
Note that the graphical peak flow and tabular hydrograph methods presented in TR-55 (and 15
associated WinTR55 computer program) cannot be directly applied to Guam transportation 16
projects, since the TR-55 methods use the NRCS standardized 24-hour rainfall 17
distributions. 18
Alternately, peak discharge using the TR-55 CN formula with the Guam one-hour storm 19
distribution (Table 4-2) can be closely approximated by computing the runoff volume (runoff 20
depth multiplied by drainage area) for short time increments during the one-hour storm 21
duration, where the amount of precipitation for each time increment is based on the fraction 22
of rainfall from Table 4-2. The easiest way to do this is by tabulating the incremental 23
calculations in a computer spreadsheet where the volume of runoff for each time period can 24
be accumulated to form a simple runoff hydrograph. Each time increment should be equal 25
in length and not longer than a few minutes. The spreadsheet can be further expanded as 26
needed to determine constraints for simple routing and required detention storage for pre- 27
versus post-development conditions (sample spreadsheets for use as design aids may be 28
available from DPW representatives on a case-by-case basis). 29
The TR-20 public domain computer program and user manual can be downloaded at: 30
http://www.wsi.nrcs.usda.gov/products/W2Q/H&H/Tools_Models/tool_mod.html 31
4.4.3 UNIT HYDROGRAPH (UH) 32
A dimensionless UH is defined as the direct runoff hydrograph resulting from a rainfall event 33
that has a specific temporal and spatial distribution, and that lasts for a unit duration of time. 34
The ordinates of the UH are such that the volume of direct runoff represented by the area 35
under the hydrograph is equal to 1 in. of runoff from the drainage area. In the development 36
of a UH, there are several underlying assumptions made, such as uniform rainfall intensity 37
and duration over the entire watershed. 38
4 - 14 August 2010
The NRCS has developed a UH procedure that has been widely used for watershed flows 1
and flood estimating purposes. The UH used by this method is based on an analysis of a 2
large number of natural UHs from a broad cross section of geographic locations and 3
hydrologic regions. This method is easy to apply. The only parameters that need to be 4
determined are the peak discharge and the time to peak. A standard UH is constructed 5
using these two parameters. 6
Procedures for estimating the runoff volume using UH methods are discussed in 7
Section 6.1 of FHWA publication HDS-2. 8
4.4.4 SANTA BARBARA URBAN HYDROGRAPH (SBUH) METHOD 9
The SBUH was developed by the Santa Barbara Flood Control and Water Conservation 10
District in California as a short cut to the NRCS UH method. The primary difference 11
between the SBUH and the NRCS UH method is the use of the unit hydrograph in 12
computing the runoff hydrograph. The SBUH replaces the unit hydrograph with an 13
algorithm. The SBUH method greatly simplifies the computation of runoff hydrographs. 14
The SBUH uses two steps to synthesize the runoff hydrograph: 15
1. Compute the instantaneous hydrograph 16
2. Compute the runoff hydrograph 17
The instantaneous hydrograph, It, in cubic feet per second (cfs), for each time step dt, is 18
computed as follows: 19
20
Where 21
Rt=total runoff depth at time increment dt (inches) 22
A=area in acres 23
dt=time interval in minutes 24
The runoff hydrograph, Qt, is then obtained by routing the instantaneous hydrograph, It, 25
through an imaginary reservoir with a time delay equal to the Tc of the drainage basin. The 26
following equation estimates the routed flow, Qt: 27
28
Where: 29
30
This procedure is best applied using a spreadsheet, described as follows: 31
32
4 - 15 August 2010
1
Column Description
1 Time step number. This is a column starting with zero and incrementing by one.
2 Time in minutes. For a time step of 10 minutes, the column would increment from zero,
10, 20, 30 etc.
3 Rainfall distribution as a percent of the total precipitation.
4 Incremental rainfall (inches). This is the total precipitation multiplied by column 3. Be
sure to divide by 100, since column 3 is in percent.
5 Accumulated rainfall (inches). Sum the previous column. (Generally, the rainfall
distribution is provided as an accumulated distribution, meaning at time zero, there is
zero rainfall and at the last time step, the rainfall is one or 100 percent accumulated. If
that is the case, column 3 can be omitted, column 4 can be the accumulated rainfall
ratio, and column 5 can be the total precipitation multiplied by column 4.)
6 Accumulated runoff calculated using the CN equation for the pervious area. This column
also checks to see if column 5 is less than 0.2s. If it is, then zero is returned; otherwise,
the abstraction is returned.
7 Incremental runoff. Subtract the previous row in column 6 from this row in column 6.
8 Accumulated runoff calculated using the CN equation for the impervious area.
9 Incremental runoff. Subtract the previous row in column 8 from this row in column 8.
10 Total runoff, which is ((PERVIOUS area / Total area) x column 7) + ((IMPERVIOUS
area / Total area) x column9).
11 Instant hydrograph. This is where the SBUH method diverges from the NRCS Unit
Hydrograph method. For the SBUH method, apply the equation:
(60.5 x column 10 x A) / dt
Where A is the total project area and dt is the time increment.
12 Design hydrograph. Apply the remainder of the SBUH method equation:
Column 12 of Previous Time Step + (w x [Column 11 of the Previous Time Step+
Column 11 of the Present Time Step - (2 x Column 12 of the Previous Time Step)])
Table 4-4: SBUH Spreadsheet Method 2
The SBUH method assumes that the impervious portion of the watershed is directly 3
connected to the drainage system. It is also important to note that, unless depression 4
storage is specified for the impervious area, the SBUH method assumes that 100 percent of 5
the rain that falls on the impervious area becomes runoff. Although the volume of 6
depression storage on an impervious surface is small, it is not always reasonable to 7
discount it. The specific magnitude of depression storage is generally not measured in the 8
field; however, depression storage depths for paved areas can range from 0.06 in. to 9
4 - 16 August 2010
0.26 in. on typical flat slopes, and the designer might want to make an estimate based on 1
their field experience. 2
The SBUH is usually available in many hydraulic modeling computer programs, being 3
particularly useful in design of off-site or more complicated roadway conveyance systems. 4
4.4.5 USGS REGRESSION EQUATIONS 5
Regional analysis methods use records for streams or drainage areas in the vicinity of the 6
watercourse under consideration that have similar characteristics to develop peak volume 7
estimates. Using recorded flood flow data, statistical analysis techniques are used to 8
determine the watercourse flood flow return frequencies from which regression equations 9
are developed for site-specific watershed, with the variables of slope, land use, precipitation 10
amount, excess rainfall, shape, and area. These regression equations can then be used for 11
estimating flood flows in similar, nearby watercourses. At present, there are limited 12
available stream flow records. 13
The U.S. Army Corps of Engineers (COE) developed regression equations based on actual 14
stream flow data (limited records prior to 1980) to use for un-gauged watersheds in 15
southern Guam. These were published in the COE’s Guam Storm Drainage Manual dated 16
September 1980. These published equations were statistically compared against more 17
recent stream flow data as part of the development of this TSDM. The equations were 18
found to still be valid. They are shown in Table 4-5. 19
20
21 22
23 24
25 26
27 28
29 30
31 32
Table 4-5: USGS Regression Equations for Guam 33
Where, Q = flow in cubic feet per second and DA = drainage area in square miles. 34
The COE 1980 Guam Storm Drainage Manual also included flows per return frequencies 35
for nine gauged streams. These are listed in Table 4-6 for reference purposes. 36
37
4 - 17 August 2010
Stream
Stream Flow in CFS
Exceedance Frequency
(%) 0.01 0.10 2 5 10 20 50 99
Recurrence Interval ()
Yrs. 1,000 100 50 20 10 5 2 1.01
Ugum River 21,316 12,419 10,244 7,604 5,887 4,316 2,370 452
Pauliluc River (Tinaga) 6,901 4,044 3,322 2,490 1,933 1,418 786 153
Almagosa Springs 1,419 862 744 573 457 348 205 48
Umatac River 16,121 10,148 8,595 6,666 5,354 4,102 2,455 594
Geus River 9,500 5,086 4,048 2,884 2,145 1,494 749 110
Finile Creek 539 418 380 332 294 254 191 88
Pago River 14,118 10,405 9,297 7,888 6,826 5,721 4,083 1,602
Ylig River 8,482 6,530 5,929 5,150 4,550 3,912 2,929 1,314
Imong River 6,103 4,592 4,147 3,547 3,100 2,631 1,920 803
Table 4-6: Frequency of Flows for Nine Rivers on Guam
4 - 18 August 2010
4.4.6 HYDROLOGIC COMPUTER PROGRAMS 1
The rapid development of computer technology in recent years has resulted in the 2
development of many mathematical models for the purpose of calculating runoff and 3
other hydrologic phenomenon. 4
Hydrograph methods can be computationally involved, so computer programs such as 5
HEC-1, HEC-HMS (developed by COE), TR-20 (developed by NRCS), StormShed and 6
HYDRAIN (developed by FHWA), are almost exclusively used to generate runoff 7
hydrographs. Many of these programs will also hydraulically route these hydrographs, 8
which is important in the design of stormwater detention, other water quality facilities, 9
and pump stations. They can also be used to evaluate conveyance flows by routing 10
through large storm drainage systems to more precisely reflect flow peaking conditions 11
in each segment of complex systems. Since the above-referenced techniques require a 12
large amount of time and data to perform basin-wide computer simulation and rainfall-13
runoff modeling, these should be reserved for larger or special projects that justify 14
applying the additional time and manpower. The additional time and cost must be 15
balanced against the need for a higher level of detail and accuracy that can be obtained. 16
As a word of caution, the designer should refrain from accepting the computer generated 17
input without questioning the reasonableness of the results obtained from a hydrologic 18
viewpoint. 19
Depending upon the amount of detail and accuracy of the input data, these models can 20
be very efficient at showing conveyance system response to particular designs and 21
assist the designer in preparing an adequate conveyance or flood protection system in 22
the most efficient and cost effective manner. 23
5 - 1 August 2010
5 HYDRAULICS ANALYSIS 1
5.1 GENERAL 2
Stormwater drainage design is an integral component of overall stormwater 3
management. Good drainage design must strive to maintain compatibility and minimize 4
interference with existing drainage patterns; control flooding of property, structures, and 5
roadways; and minimize the environmental impacts. 6
Highway drainage design is much more than the application of technical principles of 7
hydrology and hydraulics. Good drainage design is a matter of properly balancing 8
technical principles and data with the environment, while giving due consideration to 9
other factors such as safety and economics. Such design can only be accomplished 10
through the liberal use of sound engineering judgment. A goal in highway drainage 11
design should be to perpetuate natural drainage, insofar as it is practical. 12
Various types of drainage facilities are required to protect the highway against surface 13
and sub-surface water. Drainage facilities must be designed to convey the water across, 14
along, or away from the highway in the most economical, efficient, and safe manner, 15
while protecting the highway and without impacting or damaging the adjacent property. 16
The purpose of this chapter of the Guam Transportation Stormwater Drainage Manual 17
(TSDM) is to provide hydraulic analysis requirements specific to highway design on the 18
Island of Guam. 19
5.2 FREQUENCY POLICY 20
A range of runoff discharges from a range of storm recurrence intervals are used to 21
evaluate drainage facilities. The rate of runoff from a drainage area will vary depending 22
on the recurrence interval of the storm being analyzed. The recurrence interval defines 23
the frequency that a given event (e.g., rainfall or runoff) is equaled or exceeded on 24
average, once in a period of years. For example, if the 25-year frequency rainfall is 25
2.4 in., a storm event of this size or greater would be expected to occur on average once 26
every 25 years over an infinite time period. The exceedance probability, which is the 27
reciprocal of the recurrence interval of this 2.4-in. storm, would have a 0.04 probability, 28
or a 4 percent chance of occurrence in any given year. 29
Drainage frequencies that are commensurate with the relative importance of the 30
highway, associated risks, and possible damage to adjacent property should be 31
selected. When selecting a storm frequency for design purposes, consideration is given 32
to the potential degree of damage to the roadway and adjacent property; the potential 33
hazard and inconvenience to the public; the number of users on the roadway; and the 34
initial construction cost of the hydraulic structure. The way in which these factors 35
interrelate can become quite complex. The policy regarding design storm frequency for 36
typical hydraulic structures is listed in Table 5-1. Thus, the designer does not have to 37
perform a risk analysis for each structure on each project. 38
5 - 2 August 2010
Type of Structure Storm Frequency (years)
Gutters 10
Storm Drain Inlets – On Longitudinal Slope
10
Storm Drain Inlets – Vertical Curve Sag 50
Storm Drain Laterals 10
Storm Drain Trunk Lines 25
Ditches – Roadside 10
Ditches – Outfall and canals 25
Culverts 50-year for main routes and 25-year for secondary routes.
Bottomless Culverts 50
Culverts – Check for Overtopping 100
Bridges – Design for Flow Passage and Scour
100 Maintain 2 ft. clear between HW and lowest member.
For spill through abutments, try to maintain 10 ft. between top edge of channel and toe of abutment.
Main Routes
10-year: Flow spread width not to exceed shoulder and 2 ft. of the adjacent lane.
100-year: No overtopping at bridges and culverts; one lane in each direction passable for emergency vehicles. Check for flow damage and provide protection as required.
Secondary Routes 10-year: Flow spread width not to exceed half of the inner lane.
100-year: Passable for emergency vehicles. Check for flow damage and provide protection as required.
Table 5-1: Recommended Frequency for Design of Hydraulic Structures 1
The designer should carefully consider the application of the structure design storm 2
frequency as related to the overall intent of the stormwater conveyance and discharge 3
system. The interconnecting storm drainage conveyance, treatment, detention, and 4
discharge system needs to operate in a balanced condition. For example, although the 5
stipulated design frequency storm for a roadside ditch is 10 years, that may not be 6
sufficient if that ditch needs to convey flows to a detention facility requiring a 25-year 7
frequency design. The ditch needs to be able to deliver up to the peak flows that the 8
detention facility is designed for, or the overall drainage system will not function properly. 9
Likewise, a project in the limestone area of northern Guam may not have a suitable 10
overtopping discharge location requiring that the 100-year frequency storm volume be 11
5 - 3 August 2010
contained within the project limits until it can fully infiltrate. For this case, the individual 1
drainage system components may require sizing to a different performance level (i.e., 2
different from the capacity required using the design frequency shown in Table 5-1) 3
depending on their location and function within the interconnecting system. 4
5.3 ROADWAY DRAINAGE 5
Water on the pavement will slow traffic and contribute to accidents from hydroplaning 6
and loss of visibility from splash and spray, so removal of water on pavement in an 7
expeditious and efficient manner is important. Pavement drainage design will provide for 8
effective removal of water from the roadway surface. 9
Pavement drainage design is typically based on a design discharge and an allowable 10
spread of water across the pavement. Design features such as cross slope, longitudinal 11
slope, and gutter section can affect the size, type, and spacing of inlets. This Section 12
provides the fundamentals of gutter flow, inlet interception capacity, bridge deck 13
drainage, and an evaluation of the potential for hydroplaning. 14
The level of service of facilities that provide for the drainage of roadway surfaces should 15
be consistent with the level of service being provided by the roadway. Guidelines are 16
given for evaluating roadway features as they relate to pavement drainage, and for 17
selecting an appropriate design frequency. Procedures for performing gutter flow 18
calculations are based on a modification of the Manning Equation. Inlet capacity is 19
discussed for curb opening inlets, grated gutter inlets, combination inlets, slotted pipe 20
and trench drain, and bridge deck drains. 21
5.3.1 ROADWAY FEATURES 22
Roadway features considered during gutter, inlet, and pavement drainage calculations 23
include: 24
Longitudinal slope 25
Cross slope 26
Curb and gutter 27
Ditches 28
Bridge decks 29
5.3.1.1 LONGITUDINAL SLOPE 30
A minimum longitudinal gradient is important for curbed pavement because it is 31
susceptible to stormwater spread. Flat gradients on uncurbed pavement can lead to a 32
spread problem if vegetation is allowed to build up along the pavement edge. 33
Desirable gutter grades should not be less than 0.5 percent for curbed pavements, and 34
not less than 0.3 percent on very flat terrain. Minimum grades can be maintained on very 35
flat terrain through the use of warping the cross slope or rolling the profile. 36
5 - 4 August 2010
To provide adequate drainage in sag vertical curves, a minimum slope of 0.3 percent 1
should be maintained within 50 ft. of the low point of the curve. This is accomplished 2
where the length of the curve divided by the algebraic difference in grades in percent (K) 3
is equal to or less than 167. This is represented as: 4
5
6
Equation 5-1 7
Where: 8
K = Vertical curve constant in m/percent (ft./percent) 9
L = Horizontal length of curve in m (ft.) 10
Gi = Grade of roadway in percent 11
5.3.1.2 CROSS SLOPE 12
The American Association of State Highway and Transportation Officials (AASHTO) 13
policy on geometric design is standard practice and should be consulted for more 14
details. Table 5-2 indicates an acceptable range of cross slopes. These cross slopes are 15
a compromise between the need for reasonably steep cross slopes for drainage and 16
relatively flat cross slopes for driver comfort and safety. 17
18
Surface Type Range of Cross Slopes (ft. /ft.)
Travel Way
Two Lanes 0.015 - 0.025
Three or More Lanes 0.015 minimum for the two upslope lanes; increase 0.005 to 0.010 per lane downslope to a 0.04 maximum
Shoulders 0.025 - 0.060
Parking 0.005 minimum
Table 5-2: Recommended Pavement Cross Slope 19
The use of cross slopes steeper than 2 percent on pavements with a central crown line 20
is not desirable. On multi-lane roadways where three lanes or more are sloped in the 21
same direction, it is desirable to counter the resulting increase in flow depth by 22
increasing the cross slope of the outermost lanes. The two lanes adjacent to the crown 23
line should be pitched at the normal slope, and successive lane pairs, or outward 24
portions thereof, should be increased by 0.5 percent to 1 percent. The maximum 25
pavement cross slope should be limited to 4 percent. 26
5 - 5 August 2010
Additional guidelines related to cross slope are: 1
Although not encouraged, inside lanes can be sloped toward the median if 2
conditions warrant. 3
Median and off-site areas should not be drained across travel lanes. 4
The number and length of flat slope pavement sections in cross slope transition 5
areas should be minimized. Consideration should be given to increasing cross 6
slopes in sag vertical curves, crest vertical curves, and in sections of flat 7
longitudinal grades. 8
Shoulders should be sloped to drain away from the pavement, except with 9
raised, narrow medians and along superelevations. 10
5.3.1.3 CURB AND GUTTER 11
Curbing at the outside edge of pavements is normal practice for urban roadway facilities. 12
Curbs can be combined with formed concrete gutters. The gutters are generally 12 in. to 13
30 in. wide (refer to the Department of Public Work‘s [DPW‘s] standard construction 14
drawings for additional details), matching the pavement cross slope on the high side and 15
depressed with a steeper cross slope on the low side, usually at 8 percent. Typical 16
practice is to place curbs at the outside edge of the traveled way, the shoulders, or the 17
parking lanes, depending on the type of roadway. The gutter width is normally included 18
as a part of the pavement width. 19
Shoulder gutters are used to protect fill slopes from erosion caused by water from the 20
roadway pavement. Shoulder gutters are required on all fill slopes higher than 10 ft. 21
where the side slope is steeper than 3 horizontal to 1 vertical. Shoulder gutters are also 22
required at the bridge ends where concentrated flow from the bridge deck would 23
otherwise run down the slope or over a retaining wall/abutment. Shoulder gutters are 24
formed by placing a curb at the edge of the shoulder under the face of the guardrail, or 25
by concrete barriers such as those used at bridge approaches and on high fills. 26
5.3.1.4 DITCHES 27
Roadside channels and ditches are commonly used with uncurbed roadway sections to 28
convey runoff from the highway pavement and from areas that drain toward the highway. 29
Due to access and right-of-way limitations, roadside ditches cannot be used on most 30
urban arterials. They can be used in cut sections, depressed sections, and other 31
locations where sufficient right-of-way is available and driveways or intersections are 32
infrequent. 33
Curbed roadway sections are relatively inefficient at conveying water, so the area 34
tributary to the gutter section should be kept to a minimum to reduce the hazard of water 35
on the pavement. Where practicable, the flow from the major area draining toward the 36
5 - 6 August 2010
curbed pavement should be intercepted by ditches or other pipe/inlet systems, as 1
appropriate. 2
It is preferable to slope the median areas and inside shoulders to a center swale to 3
prevent drainage from the median area running across the pavement. This is particularly 4
important for facilities with two or more lanes of traffic in each direction. In some cases, 5
detention can be included in shallow medians to reduce the size of runoff facilities. 6
5.3.1.5 BRIDGE DECK 7
Bridge deck drainage is similar to that of curbed roadway sections. It is often less 8
efficient because cross slopes are flatter; parapets collect large amounts of debris; and 9
smaller drainage inlets or scuppers have a high potential for becoming clogged by 10
debris. Bridge deck constructability usually requires a constant cross slope, so the usual 11
guidelines do not apply. Because of the difficulties in providing and maintaining 12
adequate deck drainage systems, gutter flow from roadways should be intercepted 13
before it reaches a bridge. In many cases, deck drainage must be carried several spans 14
to the bridge-end disposal. 15
Zero gradients and sag vertical curves should be avoided on bridges. The minimum 16
desirable longitudinal slope for bridge deck drainage should be 0.2 percent. When 17
bridges are placed on a vertical curve and the longitudinal slope is less than 0.2 percent, 18
the gutter spread should be checked to ensure a safe, reasonable design. 19
Piped and grated scupper-type inlets are the recommended method for deck drainage 20
because they can reduce the problems of transporting a relatively large concentration of 21
runoff in an area of generally limited right-of-way. They also have a low initial cost and 22
are relatively easy to maintain. However, the use of scuppers should be evaluated for 23
site-specific concerns, and scuppers should always be piped in down drains to stable 24
and erosion-protected runoff points. Runoff collected and transported to the end of the 25
bridge should generally be collected by inlets and down drains, although paved flumes 26
may be used for minor flows in some areas. 27
5.3.2 HYDROPLANING 28
Hydroplaning occurs when a tire is separated from the roadway surface by a layer of 29
fluid. Hydroplaning is a function of the water depth, roadway geometrics, vehicle speed, 30
tread depth, tire inflation pressure, and conditions of the pavement surface. In problem 31
areas, hydroplaning may be reduced by the following: 32
Design the highway geometries to reduce the drainage path lengths of the water 33
flowing over the pavement. This will prevent flow build-up. 34
Increase the pavement surface texture depth by methods such as grooving 35
Portland Cement Concrete (PCC). An increase of pavement surface texture will 36
increase the drainage capacity at the tire pavement interface. 37
5 - 7 August 2010
The use of open-graded asphaltic pavements has been shown to greatly reduce 1
the hydroplaning potential of the roadway surface. This reduction is due to the 2
ability of the water to be forced through the pavement, under the tire. This 3
releases any hydrodynamic pressures that are created, and the potential for the 4
tire to hydroplane. 5
The use of drainage structures along the roadway to capture the flow of water 6
over the pavement will reduce the thickness of the film of water and reduce the 7
hydroplaning potential of the roadway surface. 8
5.3.3 SPREAD 9
The design frequency for pavement drainage should be consistent with the frequency 10
selected for other components of drainage systems. 11
5.3.3.1 SPREAD CRITERIA 12
Spread is dependent on the road classification, as noted in Table 5-3 below: 13
14
Road Classification Design Frequency
Design Spread
Main Routes With Shoulder
10 years Shoulder + 2 ft.
Without Shoulder
10 years Gutter + 3 ft.
Sag Location
50 years Shoulder + 2 ft.
Collector and Local Streets
With or Without Shoulders
10 years Half of Outside Driving Lane
Sag Location
50 years Half of Outside Driving Lane
Table 5-3: Allowable Spread Criteria 15
5.3.4 GUTTER FUNDAMENTALS 16
A pavement gutter is the section of a roadway normally located at its outer edge to 17
convey stormwater runoff. It may include a portion of a travel lane or be a separate 18
section at the edge of the traveled way or at the edge of the shoulder, but it usually has 19
a triangular shape, defined by the cross slope and curb. In lieu of curbs, it is possible to 20
use a V-shaped monolithic pavement section; but only when traffic control is 21
unnecessary, such as with a low-volume residential street. 22
Major components of the typical gutter section include: 23
24
5 - 8 August 2010
Pavement cross slope (Sx) 1
Longitudinal slope (SL) 2
Width of flow or spread (T) 3
Width of depressed gutter flow (W) 4
Depth of gutter flow (d) 5
Cross slope of depressed gutter (Sw) 6
Sketches showing the relationship of these components for three typical sections are 7
presented in Figure 5-1. The first sketch shows a curb and gutter section with a straight 8
cross slope; the second, a V-shaped section without a curb; and the third, a depressed 9
curb and gutter section of width (W) and cross slope (SW). These sketches are provided 10
to define fundamental parameters and are not intended to be used as the Guam 11
Department of Public Works‘ (DPW) standard details. 12
Gutter flow is a form of open-channel flow that should be analyzed using a modified form 13
of the Manning Equation. Application details and example problems can be found in 14
Section 6.2 of FHWA publication HDS 4, Introduction to Highway Hydraulics, No. FHWA-15
NHI-08-090, June 2008. 16
17
18
19
Curb with Straight Cross Slope 20
21
22
No Curb, V-Shaped Gutter 23
5 - 9 August 2010
1
2
Curb with Depressed Gutter 3
4
Figure 5-1: Typical Gutter Sections 5
5.3.4.1 FLOW IN SAG VERTICAL CURVES 6
As gutter flow approaches the low point in a sag vertical curve, the flow can exceed the 7
allowable design spread values as a result of the continually decreasing gutter slope. 8
The spread in these areas should be checked to ensure it remains within the allowable 9
limits. If the computed spread exceeds the design values, additional inlets should be 10
provided to reduce the flow as it approaches the low point. 11
5.3.5 INLET FUNDAMENTALS 12
Storm drain inlets are used to collect runoff and discharge it to an underground storm 13
drainage system. Inlets are typically located in gutter sections, paved medians, and 14
roadside and median ditches. Inlets used for the drainage of highway surfaces can be 15
divided into the following three classes: 16
Grate inlets 17
Curb opening inlets 18
Combination inlets 19
Grate inlets consist of an opening in the gutter or ditch, covered by a grate. Curb 20
opening inlets are vertical openings in the curb, covered by a top slab. Slotted inlets 21
consist of a pipe cut along the longitudinal axis with bars perpendicular to the opening to 22
maintain the slotted opening. Combination inlets consist of both a curb opening inlet and 23
a grate inlet placed in a side-by-side configuration, but the curb opening may be located, 24
in part, upstream of the grate. 25
Curb opening inlets are openings in the curb face that are generally placed in a gutter 26
section. They are more effective on flatter slopes, in sags, and with flows that typically 27
carry significant amounts of floating debris. The interception capacity of curb opening 28
inlets decreases as the gutter grade steepens. 29
5 - 10 August 2010
Combination inlets are composed of both a curb opening and a gutter type. These are 1
high-capacity inlets. The use of combination inlets will be required in sags and on grades 2
of less than 3 percent. 3
Figure 5-2 illustrates each class of inlet. For the general case, inlets should meet the 4
types included in the Standard Plans. These are grated inlets on continuous longitudinal 5
grades, and combination curb opening and grated inlets with a depression of the gutter 6
at sag locations. However, for specific approved cases, curb opening inlets without the 7
grate may be used, slotted drains may also be used with grates, and each type of inlet 8
may be installed with or without a depression of the gutter. 9
10
11 Grate Curb Opening Inlet Combination Inlet 12 13
Figure 5-2: Perspective View of Gutter and Curb Opening Inlets 14
Pavement inlets can be placed either on a continuous grade or in a sump or sag 15
condition. If pavement drainage can enter an inlet from only one longitudinal direction, a 16
continuous grade condition exists. Alternatively, if the inlet is located at a point where 17
flow enters it from two directions, a sump condition exists. 18
The interception capacity of an inlet is the gutter flow that enters an inlet under a given 19
set of conditions. The capacity changes as those conditions change. 20
The efficiency of an inlet is the percent of total gutter flow that inlet will intercept for a 21
given set of operating conditions. In mathematical form, efficiency is defined as: 22
23 24
Where: 25
E = Efficiency of an inlet in percent 26
Qi= Intercepted flow in ft.³/s 27
Q = Total gutter flow in ft.³/s 28
Flow that is not intercepted by an inlet is called bypass or carryover and is expressed 29
mathematically as: 30
5 - 11 August 2010
1 2
Where: 3
Qb = Bypass flow in ft.³/s 4
Qi = Intercepted flow in ft.³/s 5
Q = Total gutter flow in ft.³/s 6
In most cases, an increase in total gutter flow causes an increase in the interception 7
capacity of an inlet and a decrease in efficiency. 8
Pavement inlets do not provide an efficient method for collecting large quantities of 9
stormwater. Therefore, non-pavement drainage should be collected upstream of the 10
pavement, where possible. 11
Chapters 7 and 8 of FHWA publication HEC 12, Drainage of Highway Pavement, No. 12
FHWA-TS-84-202, March 1984 should be used for calculating the interception capacity 13
of inlets. 14
In accordance with the Standard Plans, generally only two types of inlets will be used for 15
curb and gutter sections. The Grated Inlet Type 1 or Type 1A will be used on continuous 16
grades. The Combination Curb and Grate Inlet-Type 2 will be used at sag locations. 17
5.3.5.1 CONTINUOUS GRADE: GRATE INLETS 18
The water flowing in a section of gutter inlet occupied by grate is called frontal flow. 19
When gutter flow velocity is low enough, the grate inlet intercepts all of the frontal flow 20
and a small portion of the side flow, which occurs along the length of the grate. As the 21
gutter flow velocity increases, water may begin to skip or splash over the grate, and the 22
efficiency of the inlet may be reduced. If splash-over does not occur, the capacity and 23
efficiency of a gutter inlet will increase with an increase of the longitudinal slope. 24
For interception calculations used for gutter spread analysis, the side flow portion should 25
be ignored, and only that portion of the gutter flow passing directly over the top width of 26
the grate will be accepted as intercepted flow. The remaining gutter flow will be 27
considered to be bypass flow. 28
5.3.5.2 SAG LOCATIONS: COMBINATION INLETS 29
Combination inlets consisting of a grate and a curb opening are required for use in sags 30
where hazardous ponding can occur. The interception capacity of the combination inlet 31
is essentially equal to that of a grate alone in weir flow, unless the grate opening 32
becomes clogged. Because of the potential for clogging, assume no less than 33
60 percent of the grate area is clogged. In orifice flow, the capacity is equal to the 34
capacity of the unclogged grate opening plus the capacity of the curb opening. 35
5 - 12 August 2010
5.3.6 INLET LOCATIONS 1
There are a number of locations where inlets may be necessary, with little regard to the 2
contributing drainage area. These locations should be marked on the plans prior to any 3
computations regarding discharge, water spread, inlet capacity, or flow bypass. 4
Examples of such locations are: 5
At all low points in the gutter grade 6
Immediately upstream of median breaks, entrance/exit ramp gores, cross walks, 7
and street intersections (i.e., at any location where water could flow onto the 8
traveled way) 9
Immediately upgrade of bridges (to prevent pavement drainage from flowing onto 10
bridge decks) 11
Immediately downstream of bridges (to intercept bridge deck drainage) 12
Within 10 ft. upgrade of cross slope reversals (i.e., the flat cross slope location of 13
super elevation transitions) 14
Immediately upgrade from pedestrian crosswalks 15
At the end of channels in cut sections 16
On side streets immediately upgrade from intersections 17
Behind curbs, shoulders, or sidewalks to drain low areas 18
In addition to the areas identified above, runoff from areas draining toward the highway 19
pavement should be intercepted by roadside channels or inlets before it reaches the 20
roadway. This applies to drainage from cut slopes, side streets, and other areas along 21
the pavement. Curbed pavement sections and pavement drainage inlets are inefficient 22
means for handling extraneous drainage. 23
In addition, it is good engineering practice to place flanking inlets on each side of the 24
low-point inlet when in a depressed area that has no outlet except through the system. 25
The purpose of the flanking inlets is to relieve the inlet at the low point if it should 26
become clogged or if the design spread is exceeded. 27
Flanking inlets can be located so they will function before water spread exceeds the 28
allowable spread at the sump location. The flanking inlets should be located so that they 29
will receive all of the flow when the primary inlet at the bottom of the sag is clogged. 30
They should do this without exceeding the allowable spread at the bottom of the sag. If 31
the flanking inlets are the same dimension as the primary inlet, they will each intercept 32
one-half of the design flow when they are located so that the depth of ponding at the 33
flanking inlets is 63 percent of the depth of ponding at the low point. Figure 5-3 below 34
illustrates the logic. 35
5 - 13 August 2010
If the flanker inlets are not the same size as the primary inlet, it will be necessary to 1
either develop a new factor or find a trial-and-error solution using assumed depths with 2
the weir equation to determine the capacity of the flanker inlet at the given depths. 3
4 Figure 5-3: Flanking Inlets 5
5.3.6.1 INLET SPACING ON CONTINUOUS GRADE 6
Inlets must be located using a trial and error procedure to maintain the depth and spread 7
of flow within allowable limits along the curb line. The last inlet (downgrade location) in a 8
series should intercept all of the flow to that point, with a bypass not to exceed 0.10 cfs 9
for the typical situation. In order to design the location of inlets on a continuous grade, 10
the computation sheet shown in Table 5-4 should be used to document the analysis. A 11
step-by-step procedure follows: 12
Step 1. Complete the blanks at the top of the sheet to identify the project, route, date, 13
and your initials. 14
Step 2. Mark on a plan the location of necessary inlets, even without considering any 15
specific drainage area, such as the locations described in Section 5.3.6. 16
Step 3. Start at a high point at one end of the job, if possible, and work toward the low 17
point. Then begin at the next high point and work backward toward the same low point. 18
Step 4. To begin the process, select a trial drainage area approximately 300 to 500 ft. 19
long below the high point, and outline the area on the plan. Include any area that may 20
drain over the curb, onto the roadway. However, where practical, drainage from large 21
areas behind the curb should be intercepted before reaching the roadway or gutter. 22
Step 5. Column 1 describes the location of the proposed inlet by number and station; 23
record this. Identify the curb and gutter type in Column 19 in accordance with the 24
Standard Plans. 25
Step 6. Compute the drainage area (acres) outlined in Step 4, and record the 26
information in Column 3. 27
5 - 14 August 2010
Step 7. Determine the runoff coefficient, C (Rational Equation) for the drainage area 1
(see Chapter 4). Select a C value or determine a weighted C value, and record the value 2
in Column 4. 3
Step 8. Compute the Time of Concentration (Tc) in minutes, for the first inlet, and record 4
it in Column 5. The Tc is the time for the water to flow from the most hydraulically remote 5
point of the drainage area to the inlet, as discussed in Chapter 4. The minimum Tc is 6
5 minutes. 7
Step 9. Using the Tc, determine the rainfall intensity from the Intensity-Duration-8
Frequency (IDF) curve for the design frequency (see Chapter 4). Enter the value in 9
Column 6. 10
Step10. Calculate the flow in the gutter using the equation Q=CIA. The flow is calculated 11
by multiplying Column 3, times Column 4, times Column 6. Enter the flow value in 12
Column 7. 13
Step 11. From the roadway profile, enter the gutter longitudinal slope, SL, at the inlet, 14
taking into account any superelevation, in column 8. 15
Step12. From the cross section, enter the cross slope, Sx, in Column 9 and the 16
Column 13 grate width, W, in Column 13. 17
Step13. For the first inlet in a series, enter the value from Column 7 into column11, and 18
because there was no previous bypass flow, enter 0 into Column 10. 19
Step14. Enter the allowable spread, as determined by the design criteria outlined in 20
Table 5-3, in Column 15. Calculate the spread, ‗T‘ for the actual gutter section using the 21
procedures discussed in Chapter 5 of FHWA publication HEC 12, and enter it into 22
Column 14. Also, determine the depth at the curb, ‗d‘, by multiplying the spread by the 23
appropriate cross slope, and enter the value in Column 12. 24
Compare the calculated spread with the allowable spread and the depth at the curb with 25
the actual curb height. If the calculated spread, Column 14, is near the allowable spread, 26
and the depth at the curb is less than the actual curb height, continue on to Step 15. 27
Otherwise, expand or decrease the drainage area up to the first inlet to increase or 28
decrease the spread, respectively. The drainage area can be expanded by increasing 29
the length to the inlet, and it can be decreased by decreasing the distance to the inlet. 30
Then repeat steps 6 through 14 until appropriate values are obtained. 31
Step 15. Select the inlet type and dimensions, and enter the values in Column 16. 32
Step 16. Calculate the flow intercepted by the grate, Qi, and enter the value in 33
Column 17. Assume that the intercepted flow is the flow prism directly over the grate 34
width. 35
Step 17. Determine the bypass flow, Qb, and enter it into Column 18. The bypass flow is 36
Column 11 minus Column 17. 37
5 - 15 August 2010
Step 18. Proceed to the next inlet down the grade. To begin the procedure, select a 1
drainage area approximately 250 to 300 ft. below the previous inlet for a first trial. 2
Repeat steps 5 through 7, considering only the area between the inlets. 3
Step 19. Compute the Tc for the next inlet based upon the area between the 4
consecutive inlets, and record this value in Column 5. 5
Step 20. Determine the rainfall intensity from the IDF curve based on the Tc determined 6
in Step 19, and record the value in Column 6. 7
Step 21. Determine the flow in the gutter, and record the value in Column 7. 8
Step 22. Record the value from Column 18 of the previous line into Column 10 of the 9
current line. Determine the total gutter flow by adding Column 7 and Column 10 10
together, and record in Column 11. 11
Step 23. Determine the spread and the depth at the curb as outlined in Step 14. Repeat 12
steps 19 through 23 until the spread and the depth at the curb are within the design 13
criteria. 14
Step 24. Select the inlet type, and record it in Column 16. 15
Step 25. Determine the intercepted flow in accordance with Step 16. 16
Step 26. Calculate the bypass flow by subtracting Column 17 from Column 11. This 17
completes the spacing design for the inlet. 18
Step 27. Repeat steps 19 through 27 for each subsequent inlet, down to the low point. 19
The bypass for the last inlet should not exceed 0.10 cfs for the typical situation. 20
21
5 - 16 August 2010
1
INLET SPACING COMPUTATION SHEET
DATE:__________________ PROJECT:_____________ROUTE:___________ Computed By: _________ Sheet _____ of _____
INLET GUTTER DISCHARGE 10 Year Design Frequency
GUTTER DISCHARGE
INLET DISCHARGE
No.
Sta
.
Dra
in.
Are
a ‗A
‘
(acre
s)
Runoff
Co
effic
ient
‗C‘
Tim
e o
f C
onc.
‗Tc‘
(min
ute
s)
Rain
fall
inte
nsity ‗I‘
(inches p
er
hour)
Q
=C
IA (
cfs
)
Long
. S
lope
SL (
ft./ft
.)
Cro
ss S
lope
Sx (
ft./ft
.)
Pre
vio
us B
ypass
Flo
w (
cfs
)
Tota
l G
utter
Flo
w (
cfs
)
Depth
‗d
‘ (i
nches)
Gra
te W
idth
‗W
‘
(feet)
Spre
ad ‗T
‘ (f
eet)
Allo
wable
Spre
ad
(feet)
Inle
t ty
pe
Inte
rcept
flo
w (
cfs
)
Bypass F
low
(cfs
)
Rem
ark
s
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19)
2
Table 5-4: Inlet Spacing Computation Sheet 3
4
5 - 17 August 2010
5.3.6.2 BRIDGE DECK DRAINAGE 1
Similarities between bridge deck and roadway drainage inlets are understandable 2
because these two types of drainage processes are, as a whole, very similar. The 3
hydraulic fundamental for the inlets are identical; however, design requirements for 4
bridge deck systems differ in the following respects from roadway systems: 5
Total or near-total interception is desirable up gradient of expansion joints 6
Deck systems are highly susceptible to inlet clogging 7
Inlet spacing is often predetermined by bent spacing 8
Inlet sizes are often constrained by structural considerations 9
To the extent possible, roadway drainage should be intercepted up gradient of a 10
bridge deck 11
The interception capacity of bridge scuppers can be generally determined as listed 12
above for curb and grated gutter inlets. See FHWA publication HEC-21 Bridge Deck 13
Drainage, May 1993 for additional design considerations. 14
5.3.7 ROADWAY GRADES 15
The critical step for establishing roadway grade lines is the determination of a roadway 16
Design High Water (DHW) elevation. The DHW provides an elevation for establishing a 17
roadway base clearance that will adequately protect the base from saturation, consistent 18
with the service requirements of the roadway. 19
The DHW for setting roadway grades is generally based on a determination of high 20
water surface for the selected storm frequency. Setting the roadway base courses above 21
this DHW level will help to avoid problems with saturation of the subgrade and 22
premature failures of the pavement. The following procedure can provide a framework 23
for such determinations: 24
a. Establish the drainage feature likely to control the water level. The following 25
features should be considered: 26
i. Published flood plain levels 27
ii. Streams and culverts. 28
iii. Canals, ditches, and swales 29
iv. Retention/detention facilities 30
v. Reservoirs 31
vi. Wetlands 32
vii. Pumping facilities- stormwater 33
5 - 18 August 2010
viii. Water table 1
ix. Local hydrogeology 2
b. Establish the service requirements for the roadway, and select an appropriate 3
design frequency for drainage facilities. Roadway classification will be either the 4
main routes or secondary roads. Use Table 5-1 to select the flood frequency for 5
various structures and conditions (i.e., 50-year for culvert crossing and 10-year 6
for roadway ditch water surface levels for main routes). 7
c. Based on the drainage feature and design frequency from Step A and Step B, 8
respectively, find the design event water surface elevation. Use procedures 9
discussed in Chapters 4 and 5 as appropriate for this analysis and determination. 10
This elevation is the DHW. 11
d. The bottom of the pavement aggregate sub-base course should be located 12
above the DHW, or alternately, the drainage conveyance systems should be 13
designed to maintain the DHW below the existing or proposed pavement sub-14
base level, by a minimum clearance level. This clearance level should be 2 to 15
3 ft. on the main routes, and no less than 1 foot on secondary roads. 16
5.3.8 CONSTRUCTION AND MAINTENANCE CONSIDERATION 17
Visual, surface-level inspections of drainage facilities should be made by maintenance 18
supervisors to identify obvious defects, hazards, or potential problems, and also to 19
monitor known problems. These inspections should be made annually and during and 20
after each major storm. Gutters should be inspected periodically and maintained to 21
permit free flow. Formed gutters should be sealed or repaired to maintain structural 22
integrity. 23
When curbs fail to perform their function due to settlement, heave, or damage, they 24
should be repaired or replaced. Curbs that are attached to sidewalks should be 25
maintained to the level of the sidewalks, approximately. 26
When the inlet is located in the clear zone, the designer should place the inlet as close 27
to parallel in the direction of traffic as possible. Placing the inlet at an angle may cause 28
an errant vehicle to overturn. 29
Quarry spalls should not be placed around inlets. This creates a safety hazard for the 30
maintenance personnel who need good footing to lift the heavy lids. If quarry spall check 31
dams are desired for erosion control, locate them a minimum of 10 ft. away from the 32
grate inlet. 33
5.4 OPEN CHANNELS AND DITCHES 34
Open-channel hydraulics provide the basis for evaluating the hydraulic capacity of most 35
roadway drainage facilities. An open channel is a watercourse that allows part of the flow 36
to be exposed to the atmosphere. This type of channel includes rivers, culverts, storm 37
5 - 19 August 2010
drain systems that flow by gravity, roadside ditches, and roadway gutters. Open-channel 1
flow design criteria are used in several areas of transportation design, including: 2
Stream and river changes 3
Flood plain analysis 4
Stream bank protection 5
Culverts and associated inlet/outlet channel revisions 6
Roadside ditches 7
Conveyance channels 8
Storm drains 9
Bridge waterways 10
Outlet discharge and downstream analysis 11
Proper design requires that open channels have sufficient hydraulic capacity to convey 12
the flow of the design storm. In the case of earth-lined channels, erosion protection is 13
required if the velocities are high enough to cause scouring. 14
5.4.1 GEOMETRIC ELEMENTS 15
Most open-channel flow problems require an evaluation of various geometric elements 16
associated with the shape of the channel. For most artificial or constructed open 17
channels, geometric elements can be determined mathematically in terms of depth of 18
flow and other dimensions for channel shape. However, for most natural channel 19
sections, profile sections based on the actual variations in the depth of flow across the 20
section are generally required. The following geometric terminology is pertinent to the 21
fundamentals of open-channel hydraulics. 22
Prismatic Channel: An artificial channel with non-varying cross section and 23
constant bottom slope. 24
Channel Section: The cross section of a channel taken perpendicular to the 25
direction of the flow. 26
Depth of Flow: The vertical distance of the lowest point of a channel 27
section from the free surface. 28
Stage: The elevation or vertical distance of the free surface above 29
a given point. 30
Top Width: The width of the channel section at the free surface. 31
Flow Area: The cross-sectional area of the flow perpendicular to the 32
direction of the flow. 33
5 - 20 August 2010
Wetted Perimeter: The length of the line of intersection of the channels wetted 1
surface on a cross-sectional plane perpendicular to the 2
direction of flow. 3
Hydraulic Radius: The ratio of the flow area to its wetted perimeter. 4
Hydraulic Depth: The ratio of the flow area to its top width. 5
Critical Flow Section Factor: The product of the flow area and the square root of the 6
hydraulic depth. 7
Uniform-Flow Section Factor: The product of the flow area and the hydraulic radius 8
raised to 2/3 power. 9
Steady Flow: Steady flow occurs in an open channel when the discharge 10
or rate of flow at any location along the channel remains 11
constant with respect to time. 12
Unsteady Flow: Open-channel flow is unsteady when discharge at any 13
location in the channels changes with respect to the time. 14
Uniform Flow: Uniform flow occurs only in a channel of a constant cross 15
section, slope, and roughness known as uniform open 16
channel. Generally, uniform flow is also steady flow. 17
Normal Depth: When the requirements of the uniform flow are met, the 18
depth of flow for a given discharge is defined as normal 19
depth. 20
Unsteady Uniform Flow: Flow in which there are variations of both space and time 21
is the most complex type of flow to evaluate 22
mathematically. 23
Rapidly Varied Flow: Also known as local phenomenon, examples of which 24
include hydraulic jump and hydraulic drop. The primary 25
example of gradually varied flow occurs when sub-critical 26
flow is restricted by a culvert or storage reservoir. The 27
water-surface profile caused by such a restriction is 28
generally referred to as a backwater curve. 29
Laminar Flow: Generally occurs when the viscous forces are strong 30
relative to inertial forces, causing water to flow in 31
streamlines. 32
Turbulent Flow: When viscous forces are weak relative to the inertial 33
forces, the flow can be classified as turbulent. 34
5 - 21 August 2010
Reynolds Number: Operational limits for laminar and turbulent flow can be 1
evaluated using a dimensionless parameter known as the 2
Reynolds Number, which is mathematically expressed as: 3
4 5
Equation 5-2 6
Where: 7
Re = Reynolds Number 8
V= Average velocity of flow in ft./s 9
L = Characteristic length in feet (pipe diameter for conduit) 10
= Kinematic viscosity of fluid in ft.2/s 11
For pipe flow, laminar flow generally occurs until the 12
Reynolds Number value exceeds approximately 2,000. A 13
transitional zone is then observed between laminar and 14
turbulent flow. The Reynolds Number value at the upper 15
limit of this transitional zone may be as high as 50,000. If 16
the pipe diameter is converted to hydraulic radius for 17
representing characteristic length in the Reynolds Number 18
value, the corresponding transitional range is 500 to 19
12,500. 20
Froude Number: The importance of gravity as a driving force in open- 21
channel drainage systems makes its effect on the state of 22
flow a major factor for evaluation. This can be done using a 23
dimensionless parameter known as the Froude Number, 24
which is expressed mathematically as: 25
26 27
Equation 5-3 28
Where: 29
FR = Froude Number 30
V = Average velocity of flow in ft./s 31
5 - 22 August 2010
L = Characteristic length in ft. (hydraulic depth for open 1
channels) 2
g= Acceleration due to gravity in 32 ft./s2 3
If the Froude Number value is greater than one, inertial 4
forces dominate, and the flow is super-critical. The flow in 5
this regime will also tend to be turbulent and unsteady for 6
the general case. If the Froude Number is less than one, 7
gravity forces dominate, and the flow is called sub-critical 8
or tranquil flow. When the Froude Number is equal to one, 9
then flow is defined as the critical flow. 10
5.4.2 CHANNEL DESIGN CRITERIA 11
Channels and ditches should be designed to convey the required frequency storm event 12
runoff with a 0.5-foot freeboard. The preferred cross section of a ditch is trapezoidal; 13
however, a V-ditch can also be used where right-of-way is limited and/or the design 14
requirements can still be met. The depth of channel should be sufficient to remove the 15
water while maintaining the required design water surface clearance below the 16
pavement sub-base. 17
Side slopes can vary but should be limited to no steeper than 2 horizontal to 1 vertical 18
for earth channels with a grassed lining. Roadside ditch side slopes along the roadway 19
should meet the clear zone design requirements and be limited to less than a 2-foot 20
design depth, unless guardrail or barrier traffic protection is provided. 21
5.4.2.1 CHANNEL TYPES 22
Open channels can be generally classified as those that occur naturally, are man-made, 23
or improve natural channels. Man-made channels are also called artificial channels, 24
including the following types, in use on most roadway projects: 25
Roadside ditch 26
Interceptor ditch 27
Swale 28
Median ditch (swale) 29
Outfall ditch 30
Lateral ditch 31
Canal 32
Each of these channel types are artificial systems designed to provide specific drainage 33
capacities. In general, roadside and median ditches are relatively shallow trapezoidal 34
channels, while swales and interceptor ditches are shallow triangular channels. These 35
5 - 23 August 2010
types are designed to handle local surface runoff from the roadway surfaces or to lower 1
the water table elevations by intercepting groundwater. 2
In most cases, outfall ditches or canals are designed as receptors of runoff from 3
numerous secondary drainage facilities, such as side ditches or storm drains. The use of 4
a roadside ditch as an outfall ditch is not recommended because its probable size and 5
depth could create a potential driving hazard. 6
5.4.2.2 DESIGN METHODS 7
The fundamental relationship for performing open-channel capacity calculations will be 8
the Manning Equation. All ditches and channels should be designed using the methods 9
and procedures in FHWA publication HDS 4. All channel designs should use the ―stable 10
channel concepts‖ in accordance with Chapter 5 of HDS 4. 11
5.4.2.3 LININGS 12
Channels should be designed to be stable under the design flow conditions. To maintain 13
the integrity of the channel, earth ditches are usually lined with grass; however, this type 14
of lining is only acceptable for flow velocities less than 5 ft. per second and where the 15
channel is normally dry. For higher velocities with a steeper hydraulic gradient and 16
channel slopes, more protective channel linings may be required. 17
The three main classifications of open-channel linings are vegetative, flexible, and rigid. 18
Vegetative linings include grass with mulch, sod, and lapped sod. Rock riprap and 19
geotextile or interlocking concrete grids are types of flexible linings, while rigid linings 20
include concrete, asphalt, sand/cement riprap, and soil cement. 21
Velocity limitations for artificial open channels should be consistent with stability 22
requirements for selected channel linings. Typical guideline velocities for these linings 23
are summarized in Table 5-5. 24
25
Lining Type Maximum Velocity (feet per second)
Grass with mulch Less than 4.0
Sod (well-maintained) 4.0
Lapped sod 5.5
Riprap 6.0
Asphalt 8.0
Geotextile grid 4.0- 8.0*
Rigid 10.0**
*Varies with grid type 26
5 - 24 August 2010
** Higher velocities acceptable with provision for energy dissipation 1
Table 5-5: Permissible Velocities for Various Linings 2
Linings should be designed using a shear stress approach, and using the calculation 3
guidelines and procedures in Chapter 5 of FHWA publication, HDS 4. 4
5.4.3 CONSTRUCTION AND MAINTENANCE CONSIDERATION 5
The design of an open channel should be consistent with DPW‘s standard construction 6
and maintenance practices. If special provisions are warranted, this information should 7
be presented in the design documents. 8
To prevent cracking and failure, concrete linings must be placed on a firm, well-drained 9
foundation. In saturated soils, empty channels with a rigid lining such as concrete, may 10
float or break up due to the buoyancy of displaced water. The total upward force on an 11
empty channel is equal to the weight of the displaced water. To prevent this, the lining 12
should be increased in thickness to add weight, or if the flow is sub-critical, weep holes 13
may be placed at intervals in the channel bottom to relieve the upward pressure on the 14
channel. When flow is super-critical, sub-surface drainage should be used instead of 15
weep holes, with properly-designed filters to avoid soil piping through the sub-drains. 16
Open channels rapidly lose their effectiveness unless they are adequately maintained. 17
Maintenance includes repairing erosion damage, mowing grass, cutting brush, and 18
removing any sediments or debris from the channel. 19
Ditches, outfalls, and detention areas must be provided, with maintenance access. 20
5.5 STORM DRAINS 21
A storm drain is the portion of the highway drainage system that receives surface water 22
through inlets and conveys the water through conduits to an outfall. It is composed of 23
different lengths and sizes of pipe or conduit connected by appurtenant structures. A 24
section of conduit connecting one inlet or appurtenant structure to another is termed a 25
"segment" or "run." The storm drain conduit is most often a circular pipe, but it can also 26
be a box or other enclosed conduit shape. Typical structure types include inlets, catch 27
basins, manholes, cleanouts, junction chambers, headwalls, flow controls, energy 28
dissipaters, and outlets. 29
All storm drain designs should be based on an engineering analysis that takes into 30
consideration runoff rates, pipe flow capacity, hydraulic grade line, soil characteristics, 31
pipe strength, potential construction problems, and potential runoff treatment issues. The 32
majority of time spent on a storm drain design is calculating runoff from an area and 33
designing the enclosed conveyance system to carry the flow. A storm drain design may 34
be performed by hand calculations or by one of several available computer programs 35
and spreadsheets. See Sections 4.4.6 and 5.5.4 for general information on hydraulic 36
modeling programs. 37
5 - 25 August 2010
Runoff flows are determined using the procedures in Chapters 2 and 4 of this TSDM. 1
The storm drain hydraulic design should follow the guidelines and procedures in 2
Chapters 7 and 8 of FHWA publication HDS 4. 3
5.5.1 DESIGN FEATURES 4
Along with determining the required pipe sizes for flow conveyance, storm drain system 5
design incorporates the following features: 6
1. Soil Conditions — Soil with adequate bearing capacity must be present to interact 7
with the pipes and support the load imparted by them. Surface and sub-surface drainage 8
must be provided to ensure stable soil conditions. Soil resistivity and pH must also be 9
known so the proper pipe material can be used. Pipe installation in poor soil conditions 10
should be performed in accordance with the review and recommendations of a qualified 11
Geotechnical Engineer, to determine additional soil treatments and bedding 12
improvements that may be required. 13
2. Inlet Spacing and Capacity — Design guidelines are detailed in Section 5.3.5. 14
3. Junction Spacing — Junctions (catch basins, grate inlets, and manholes) should be 15
placed at all breaks in grade and horizontal alignment. Pipe runs between junctions 16
should not exceed 300 ft. for pipes smaller than 48 in. in diameter, and 500 ft. for pipes 17
48 in. or larger in diameter. When grades are flat, pipes should be small or there could 18
be debris issues. Designers should consider reducing the minimum spacing. 19
4. Future Expansion — If it is anticipated that a storm drain system may be expanded 20
in the future, provisions for the expansion should be incorporated into the current design. 21
Additionally, the existing system should be inspected for structural integrity and hydraulic 22
capacity prior to expansion. 23
5. Velocity — The velocity of flow should be 3 ft. per second or greater to prevent the 24
pipes from clogging due to siltation. Velocity of flow should not be excessively high, 25
because high flow velocities (approaching and above 10 ft. per second) produce very 26
large energy losses in the storm drain system and also cause excessive wear and 27
damage to the pipes and structures due to accelerated abrasion wear and surge 28
impacts. The velocity should be calculated under full flow conditions, even if the pipe is 29
only flowing partially full with the design storm. 30
The designer should obtain separate approval for any pipes having velocities exceeding 31
10 ft. per second. The high velocity pipe designs will require features such as restrained 32
joints, thrust blocks, pipe-to-wall thrust collars, special bedding, bedding restraints, pipe 33
types resistant to abrasion damage, energy dissipaters, etc. 34
6. Grades at Junctions — Pipe crowns of branch or trunk lines entering and exiting 35
junctions should be at the same elevation. If a lateral is placed so its flow is directed 36
against the main flow through the manhole or catch basin, the lateral invert must be 37
5 - 26 August 2010
raised to match the crown of the inlet pipe. (A crown is defined as the highest point of 1
the internal surface of the transverse cross section of a pipe.) 2
7. Minimum Pipe Diameter — The minimum pipe diameter should be 12 in., except that 3
pipe located under roadway travel lanes should be 18 in. minimum. 4
8. Maximum Pipe Diameter – Designers should verify the maximum available pipe 5
diameters per manufacturing type and as suitable for the bedding and cover conditions. 6
Designers should make sure that their selected structure types and sizes are suitable for 7
the size and type of pipe to be attached to it. 8
9. Energy Losses — Energy losses are calculated to determine the hydraulic grade 9
line. The hydraulic grade line should be established as part of all storm drain systems 10
(other than capacity checks for simple layout systems involving one or two pipes). 11
Energy losses can be significant and of concern for the following situations: 12
High flow velocities in the system 13
Maintaining sufficient velocity in pipes on flat slopes 14
When inlet and outlet pipes form a sharp angle at junctions 15
When there are multiple flows entering a junction 16
When pipes entering and leaving the junction are very shallow 17
High water surface elevations at an inlet and/or outlet 18
When the hydraulic grade line is higher than the pipe profile 19
10. Outfalls — An outfall is where the storm drain system discharges to the natural or 20
existing waterway. Typically, the outfall should be designed to minimize disturbance to 21
streams and wetlands and include erosion protection. 22
Additional considerations for outfalls include energy dissipaters and tidal gates. Energy 23
dissipaters prevent erosion at the storm drain outfall. Energy dissipation should be in 24
accordance with the guidelines of Chapter 10 of FHWA publication HDS 4. Another 25
reference for the design of energy dissipaters is FHWA publication HEC No. 14, 26
Hydraulic Design of Energy Dissipaters for Culverts and Channels, No. FHWA-NHI-06-27
086, July 2006. 28
The installation of tide gates may be necessary when the outfall is in a tidal area to help 29
limit backflow through the pipe system. Tide gate outfalls need to be sized (diameters 30
and storage bay sizes) using a mass flow procedure. An older, but still available 31
reference on sizing of tide gates is the USDA National Resources Conservation Service 32
(NRCS) Technical Release No. 1, July 28, 1955. 33
In all cases, the flow velocity at the outlet of the dissipater structure should be non-34
erosive for the existing conditions, and be less than the velocity in the natural/existing 35
5 - 27 August 2010
channel. When discharging to natural streams and wetlands, consideration should be 1
made to discharge using a level spreader trench located parallel to the wetland or 2
stream vegetated buffer. The spreader trench should be designed as a combination 3
infiltration trench for low flows and to produce non-erosive surface sheet flow for larger 4
storm events. 5
11. Location — Medians and outer edges of shoulders usually offer the most desirable 6
storm drain location. In the absence of medians or shoulders, a location beyond the 7
edge of pavement on right-of-way or on easements is preferable. When a storm drain is 8
placed beyond the edge of the pavement, it is generally recommended that a one-trunk 9
system with connecting laterals be used instead of running two separate trunk lines 10
down each side of the road. 11
12. Confined Space and Structures — Any structure (catch basin, manhole, inlet, 12
junction chamber, etc.) more than 4 ft. in depth is considered a confined space. As such, 13
any structure exceeding 4 ft. in depth that could be accessed by personnel must be 14
sized with room for personnel to maneuver and stand up. This requires a minimum 15
inside dimension of 4 ft. wide by 6 ft. high. All confined spaces should meet OSHA 16
regulations for ventilation and access. Structures more than 15 ft. deep should be 17
avoided due to the limitations of cleaning by suction and rodding maintenance trucks. 18
Any design requiring a structure deeper than 15 ft. must have specific review and 19
approval on the access and maintenance aspects. 20
5.5.2 HYDRAULICS OF STORM DRAIN SYSTEMS 21
The hydraulic design of storm drainage systems requires an understanding of basic 22
hydrologic and hydraulic concepts and principles, which include flow classification, the 23
conservation of mass, the conservation of momentum, and the conservation of energy. 24
FHWA publication HDS 4 will be the primary reference used for design reviews by DPW. 25
5.5.3 STORM DRAIN DESIGN: MANUAL METHOD 26
It is recommended that storm drain systems be designed using one of the readily-27
available computer modeling programs that ―route‖ the runoff flows through the system. 28
Basic pipe system designs using handheld calculator methods will be acceptable if 29
documented similar to Table 5-6. The design using Table 5-6 should be further 30
supported with exhibits showing the layouts, contributory drainage boundaries, contours, 31
runoff travel paths, outfalls, and existing/proposed topography. Table 5-6 has five 32
divisions -- location, discharge, drain design, drain profile, and remarks, as follows: 33
Location 34
The location section provides all of the layout information for the drain. 35
Column 1 gives a general location reference for the individual drain lines, typically by 36
the name of a street or a survey line. 37
5 - 28 August 2010
Columns 2 and 3 show the stationing and offset of the inlets, catch basins, or manholes 1
— either along a roadway survey line or along a drain line. 2
Discharge 3
The discharge section presents the runoff information and total flow into the drain. 4
Column 4 is used to designate the various drainage areas that contribute to a particular 5
point in the drain system. The drainage areas should be numbered or lettered according 6
to some reference system on the drainage area maps. The type of ground cover 7
(pavement, median, etc.) may be indicated. Because drainage areas must be subdivided 8
according to soil and ground cover types, a drainage area may have several different 9
parts. 10
Column 5 shows the cumulated drainage area used for the associated pipe section 11
design in acres. 12
Column 6 shows the Rational Equation runoff coefficient (see Chapter 4). Each 13
individual drainage area must have a corresponding runoff coefficient. 14
Column 7 is the product of columns 5 and 6. Column 7 is also the effective impervious 15
area for the subsection. 16
Column 8. is the summation of the product of the coefficient ‗C‘ and the drainage area 17
‗A‘ of all of the effective areas contributing runoff to the point in the system designated in 18
Column 2. All of the individual CxA values in Column 7 contributing to a point in 19
Column 2 are summed. 20
Column 9 shows the Tc to the structure indicated in Column 2. Generally, the time 21
chosen here would be the longest time required for water to travel from the most 22
hydraulically remote part of the storm drain system to this point. This would include flow 23
over the drainage basin and flow through the storm drain pipes. The Tc should be 24
expressed to the nearest minute and should never be less than five minutes. 25
When the runoff from a drainage area enters a storm drain and the Tc of the new area is 26
shorter than the accumulated Tc of the flow in the drain line, the flow in the downstream 27
pipe should be calculated using the overall accumulated Tc with the accumulated areas 28
(both new and upstream areas combined). 29
The easiest method for determining the Tc of the flow already in the system (upstream of 30
the structure in Column 2) is to add the Tc from Column 9 of the previous run of pipe 31
(this value should be on the row above the row that is currently being filled in) to the time 32
it took the flow to travel through the previous run of pipe. To determine the time of flow 33
(or more precisely, the travel time) in a pipe, the velocity of flow in the pipe and the 34
length of the pipe must be calculated. Velocity is computed using the Manning Equation 35
and is found in Column 16 of the previous run of pipe. The length used is the value 36
5 - 29 August 2010
entered in Column 18 for the previous run of pipe. Obviously, this calculation is not 1
performed for the very first (most upstream) run of pipe in a storm drain system. 2
T1 = L/(60V) 3
Where: 4
T1 = Tc of flow in pipe in minutes 5
L = Length of pipe in ft. (m), Column 18 6
V = Velocity in ft./s (m/s), Column 16 of the previous run of pipe 7
The designer should note that this calculation assumes that the pipe is flowing full. It is 8
accurate for pipes flowing slightly less than half full up to completely full. It will be slightly 9
conservative for Tc calculations when the pipe is flowing significantly less than half full. 10
Column 10 shows the rainfall intensity corresponding to the time indicated in Column 9 11
and the location of the project. 12
The intensity is in inches per hour to the nearest hundredth. The rainfall intensity used is 13
a 25-year recurrence interval for storm drain laterals and trunks and the 10-year 14
recurrence interval for laterals without trunks. See Chapter 4 for the related intensity 15
charts. 16
Column 11 shows the amount of runoff to the nearest 10th of a cubic foot per second, 17
up to the point indicated in Column 2. It is computed as the product of columns 8 and 10. 18
This is simply applying the Rational Method to compute runoff from the entire drainage 19
area upstream of the pipe being analyzed. 20
Column 12 shows any flow, other than the runoff calculated in Column 11, to the 21
nearest 10th of a cubic foot per second that is entering the system up to the point 22
indicated in Column 2. It is rare to have flow entering a system other than runoff from the 23
drainage basin, but this does occur. For instance, this occurs when an underdrain that is 24
draining groundwater is connected to the storm drain. The label for this column indicates 25
that these flows are considered constant for the duration of the storm, so they are 26
independent of the Tc. 27
This column is also used when the junction is a drywell and a constant rate of flow is 28
leaving the system through infiltration. When this occurs, the value listed in Column 12 is 29
negative. See Chapter 6 for drywell design criteria. 30
Column 13 is the sum of columns 11 and 12 and shows the total flow in cubic feet per 31
second to the nearest 10th to which the pipe must be designed. 32
Drain Design Section 33
This section presents the hydraulic parameters and calculations required to design storm 34
drain pipes. 35
5 - 30 August 2010
Column 14 shows the pipe diameter in feet. This should be a minimum of 1 foot. Pipe 1
sizes should never decrease in the downstream direction. 2
The correct pipe size is determined through a trial-and-error process. The engineer 3
selects a logical pipe size that meets the minimum diameter requirements and a slope 4
that fits the general slope of the ground above the storm drain. The calculations in 5
Column 17 are performed and checked against the value in Column 13. If Column 17 is 6
greater than or equal to Column 13, the pipe size is adequate. If Column 17 is less than 7
Column 13, the pipe does not have enough capacity and must have its diameter or slope 8
increased, after which Column 17 must be recalculated and checked against Column 13. 9
Column 15, Pipe Slope, is expressed in feet per foot. This slope is normally determined 10
by the general ground slope but does not have to match the surface ground slope. The 11
designer should be aware of buried utilities and obstructions that may conflict with the 12
placement of the storm drain. 13
Column 16 shows the full-flow velocity. It is determined by the Manning Equation, which 14
is shown below. The velocity is calculated for full-flow conditions, even though the pipe is 15
typically flowing only partially full. Partial flows will be very close to the full-flow velocity 16
for depths of flow between 30 percent and 100 percent of the pipe diameter. 17
18
19
20 21
22 23
Velocities should be within a range of 3 to 10 ft. per second. If higher than 10 ft. per 24
second, additional design considerations must be taken into account. Velocities below 25
3 ft. per second will create additional maintenance considerations. 26
Column 17, Pipe Capacity, shows the amount of flow in cubic feet per second, which 27
can be taken by the pipe when flowing full. It is computed using the following formula: 28
Q = A V 29
Drain Profile 30
Columns 18 through 23, the Drain Profile Section, include a description of the profile 31
information for each pipe in the storm drain system. They describe the pipe profile and 32
the ground profile. The ground elevations should be finished elevations, to the 100th of a 33
foot. The items in this section are generally self-explanatory, except for Column 18. The 34
length shown is the horizontal projection of the pipe, in feet, from the center to the center 35
of appurtenances. Generally, profiles should be set to provide a minimum of 2 ft. of 36
cover over the top of the pipe. 37
5 - 31 August 2010
Remarks 1
Column 24, Remarks, is for any information that might be helpful in reviewing the 2
calculations. This space should note unique features such as drop manholes or energy 3
dissipaters, reasons as to non-conventional slopes or grade changes, specific types of 4
pipes due to loading and cover depths, velocity or maintenance considerations, changes 5
in design frequency, etc. 6
7
5 - 32 August 2010
STORM DRAIN COMPUTATION SHEET
DATE__________________ SP_____________ROUTE___________ Computed By __________________ Sheet ______ of _________
Dra
in locate
d o
n.
Fro
m S
tation
To S
tation
Contr
ibuting
Are
as
(ac.)
Cum
. D
rain
age
Are
a ‗A
‘ (a
c.)
Runoff
Co
effic
ient
‗C‘
CA
ΣC
A
Tc
(min
.)
Rain
fall
Inte
nsity ‗I‘
(in./hr.
)
Runoff
Flo
w (
cfs
)
Con. In
flow
(cfs
)
Tota
l F
low
(cfs
)
Wid
th
Pip
e D
iam
ete
r (f
t.)
Pip
e S
lope (
ft./
ft.)
Flo
w v
elo
city (
ft./
s)
Pip
e c
ap
acity (
cfs
)
Leng
th (
ft.)
Ele
va
tio
n C
han
ge
(ft.)
U/S
Gro
und E
lev.
(ft.)
D/S
Gro
und E
lev.
(ft.)
U/S
Invert
Ele
v. (f
t.)
D/S
Invert
Ele
v. (f
t.)
Rem
ark
s
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24)
Table 5-6: Storm Drain Calculation Sheet 1
2
5 - 33 August 2010
5.5.4 COMPUTER ANALYSIS 1
Storm drain design by computer analysis offers some distinct advantages over calculations 2
performed by hand. Chief among these advantages is the ability to perform complex flow 3
routings through the system, which can take into account the detention due to storage 4
within the pipes and structures and overlapping of various basin runoff hydrographs. This 5
analysis develops a more accurate picture of the hydraulic grade lines and flow conditions 6
within a storm drain system for the design storm peak conditions. 7
Most computer programs will have the ability to process various runoff hydrograph 8
calculation methods that use the Rational Method or NRCS curve numbers, as well as the 9
ability to enter storm distributions and rainfall intensity data unique to a specific location. 10
They also will allow selection of different routing methods. Most of these programs will have 11
sub-routines for designing detention within a system or accepting the input of additional 12
detention storage and discharge controls. The designer should develop a full understanding 13
of the various runoff and routing methodologies and in particular, select those that are best 14
for the application when using these programs. 15
There are several commercially-available computer programs for storm drain design. Two 16
programs accepted by DPW are StormShed by Engenious Systems, Inc. and FlowMaster 17
by Haestad Methods. 18
5.5.5 PIPE MATERIALS FOR STORM DRAINS 19
There are various pipe materials that are acceptable for storm drains. Pipe material 20
selection includes hydraulic characteristics, site conditions, geologic conditions, corrosion 21
resistance, safety considerations, and cost. Pipe types appropriate for a project are 22
dependent on several factors, including but not limited to fill height, size, strength, 23
corrosion, abrasion potential, debris passage, and necessary end treatments. 24
Pipe design and installation is based on two general categories -- rigid and flexible pipes. 25
The designer should be familiar with the design theories for both. The pipe type selection 26
and installation methodology used should be prepared using the appropriate design 27
principle. 28
Pipe selection should be made in accordance with the following constraints. 29
5.5.5.1 PIPE MATERIALS 30
Pipe materials used by DPW can be categorized into three main categories -- storm sewer 31
pipe (or otherwise called storm drain pipe), underdrain pipe, and culvert pipe. 32
Storm sewer pipe usually ranges from 12 to 48 in. in diameter, and is used to convey 33
roadway runoff or groundwater away from the roadway profile. Types of storm sewer pipes 34
and materials should be selected based on the corrosion resistance and strength 35
requirements suitable for the specific requirements. Pipe types can include concrete, 36
reinforced concrete, metal, corrugated metal, and thermoplastic solid and corrugated wall. 37
5 - 34 August 2010
All storm sewer pipes should use gasketed, leak-proof joints, and be pressure-tested after 1
installation to indicate the presence of leaking seams or joints. The design life for storm 2
sewer pipes should generally match the design life of the roadway pavement, or a minimum 3
of 25 years. If the pipe is to be installed in deep fills crossing the roadway section, then the 4
design life should be the same as required for culvert pipe. 5
Underdrain pipe is smaller diameter, perforated pipe intended to intercept groundwater and 6
convey it away from areas such as roadbeds or from behind retaining walls. Typical 7
underdrain applications use 6- to 8-in. diameter pvc, or polyethylene pipe, both solid wall 8
and corrugated, but larger diameters can be specified. Underdrain pipe is generally used in 9
conjunction with well-draining backfill material and a geotextile filter fabric. The design life of 10
underdrain pipe should be the same as required for storm sewer pipe. 11
Culvert pipe is a conduit under a roadway or embankment used to maintain flow from a 12
natural channel or drainage ditch. Culverts are often located under high fills and usually 13
cross the roadway making replacement difficult. Because of this, a minimum design life 14
expectancy of 50 year is required for all culverts under the main route roadways. A 15
minimum 25-year design life should be used for other secondary road locations. The type of 16
culvert selected can vary substantially from one location to another due to the hydraulic 17
requirements, loadings, and corrosion resistance. Culverts use storm sewer pipe, reinforced 18
concrete box sections (both rectangular and bottomless), corrugated metal, corrugated 19
plastic pipe, corrugated metal pipe arch, and structural plate. 20
5.5.5.2 PIPE DURABILITY 21
Once a designer has determined the pipe classification needed for an application, the next 22
step is to ensure that the pipe durability will extend for the entire design life. There are two 23
main durability factors -- corrosion potential and abrasion resistance. 24
Corrosion potential will be categorized generally as moderate or extreme. If the soil or water 25
pH is less than 5 and greater than 8.5 and/or with a soil resistivity less than 1,000 ohm-cm 26
then there is a potential for extreme corrosion conditions. All pipe installations in the coastal 27
environment or brackish water conditions can also be considered as extreme corrosion 28
conditions. If the pH and soil resistivity of the installation site is not known, then testing 29
should be performed and pipe material selection made accordingly. 30
Pipes suitable for the moderate condition include thermoplastic, concrete, and metal. Metal 31
pipes should have protective treatments. These treatments are typically protecting on both 32
sides with asphaltic or polymer coatings. Where abrasion of the bottom of pipes due to high 33
velocities and heavy bed loads is a problem, then additional coatings such as asphalt invert 34
paving is required. Corrugated steel pipe polymer coatings should conform to AASHTO M-35
246, composed of a minimum of 10 mils thick polyethylene and acrylic acid copolymer. 36
Asphaltic coatings are usually a proprietary manufacturing process, using a spun asphalt 37
lining on the inside for a hydraulically-smooth interior. Galvanized metal is not suitable due 38
to its potential for loading of zinc into the environment. An alternative to asphalt protective 39
5 - 35 August 2010
treatments for corrugated steel pipes is to increase the wall thickness to compensate for 1
loss of metal for the desired life span. 2
Pipes suitable for high corrosion potential conditions are thermoplastic and concrete pipes 3
only. 4
Abrasion is the wearing away of pipe material by water-carrying sands, gravels, and rocks. 5
All types of pipe material are subject to abrasion and can experience structural failure 6
around the pipe invert if not adequately protected. Particular concern would be for streams 7
with high bed loads, and with frequent or steady low flows with velocities 6 to 10 ft. per 8
second. For these conditions, consideration should be given to placing the culverts on flat 9
grades to encourage deposition within the culvert; reducing velocity through the culvert by 10
increasing diameters and flattening grades; providing extra asphaltic paving of the inverts; 11
and increasing wall thickness by at least one or two standard gauges on metal pipes. 12
Where velocities are greater than 10 ft. per second, asphaltic coatings will have extremely 13
short life expectancies and the wear on concrete inverts can be quite rapid depending on 14
the amount of abrasives in the flow. For these conditions, the use of thermoplastic pipe with 15
restrained joints is recommended. Solid wall High Density Polyethylene (HDPE) pipe with 16
fusion welded joints is probably the most resistant to the abrasion forces. The use of 17
continuous wall HDPE requires additional considerations for the thermo and dynamic forces 18
involved, such as pipe anchors, wall flanges, and surge blocks. 19
Pipes that have deteriorated over time due to either corrosion or abrasion will significantly 20
affect the structural integrity of the roadway embankment. Once identified, these pipes 21
should be replaced as soon as possible. In particular, they should be replaced as part of a 22
new project‘s overall construction. Failure of a pipe will usually result in the ultimate failure 23
of a roadway. 24
5.5.5.3 FILL HEIGHT 25
Pipe types are constrained by their ability to carry the loadings from live vehicular traffic and 26
longer-term, dead loads from soil cover. Assuming that the pipe is installed in accordance 27
with DPW‘s standards, the pipe should perform satisfactorily if placed within the limitations 28
shown in the Fill Height Tables 5-7a through 5-7c. 29
Pipe systems should be designed to provide at least 2 ft. of cover over the pipe measured 30
from the outside diameter of the pipe to the bottom of the pavement. This measurement 31
does not include any asphalt or concrete paving above the top course. This depth tends to 32
provide adequate structural distribution of the live load and also allows a significant number 33
of pipe alternatives to be specified on a contract. 34
During construction, these minimum cover requirements may not be adequate to protect the 35
pipe from equipment loadings, where additional cover or protected crossings may be 36
required. 37
38
5 - 36 August 2010
Concrete Pipe
Diameter (inches)
Maximum Cover in Feet
Plain AASHTO
M 86
Class II AASHTO
M 170
Class III AASHTO
M170
Class IV AASHTO
M 170
Class V AASHTO
M 170
12 18 10 14 21 26
18 18 11 14 22 28
24 16 11 15 22 29
30 11 15 23 29
36 11 15 23 29
48 12 15 23 29
60 12 16 24 30
72 12 16 24 30
84 12 16 24 30
Table 5-7a: Fill Height, Concrete Pipe 1
2
Concrete Pipe
Diameter (inches)
Pipe Wall Thick.
(inches)
Minimum Cover in Feet
Plain AASHTO
M 86
Class III AASHTO
M170
Class IV AASHTO
M 170
Class V AASHTO M
170
12 2 1.5 1.5 1.0 0.5
18 2.5 1.5 1.5 1.0 0.5
24 3 1.5 1.5 1.0 0.5
30 3.5 1.5 1.5 1.0 0.5
36 4 1.5 1.5 1.0 0.5
48 5 1.5 1.0 0.5
60 6 1.5 1.0 0.5
72 7 1.5 1.0 0.5
84 8 1.5 1.0 0.5
Table 5-7b: Concrete Pipe for Shallow Cover Installations 3
4
Solid Wall PVC Profile Wall PVC Corrugated Polyethylene
ASTM D 3034 SDR 35 3 in. to 15 in. dia.
ASTM F 679 Type 1 18 in. to 48 in. dia.
AASHTO M 304 or
ASTM F 794 Series 46 4 in. to 48 in. dia.
AASHTO M 294 Type S 12 in. to 60 in. dia.
25 ft. All diameters
25 ft. All diameters
15 ft. All diameters
Table 5-7c: Thermoplastic Pipe, Fill Height 5
Minimum and maximum fill heights for corrugated metal pipe should be based on loading 6
calculations established for flexible pipe theory in accordance with the pipe manufacturer‘s 7
recommendations for that specific size, corrugation type/size, material type, and wall 8
thickness. 9
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5.5.6 SUB-SURFACE DRAINAGE 1
Sub-surface drainage is provided for the control of groundwater encountered at roadway 2
locations. Groundwater, as distinguished from capillary water, is free water occurring in a 3
zone of saturation below the ground surface. The sub-surface discharge depends on the 4
effective hydraulic head and on the permeability, depth, slope, thickness, and extent of the 5
aquifer. 6
Solving sub-surface drainage problems often calls for specialized knowledge of geology 7
and the application of soil mechanics. The designer should work directly with the 8
Geotechnical Engineer regarding the final design of underdrains. 9
Typical underdrain installations would be those provided for control of seepage in cuts or 10
side hills, or the lowering of the groundwater table for proper subgrade drainage. Sub-11
surface drainage pipe size should be determined from the Geotechnical Engineer‘s 12
recommendations, or based on the area of interception, the permeability of the contributing 13
soil layers, and most likely, the internal soil hydraulic grade line. When sub-surface 14
drainage is connected to a storm drain system, the invert of the underdrain pipe should be 15
placed above the operating water level in the storm drain. This helps to prevent reverse 16
flow into the underdrain and in particular flooding of pavement sub-base layers for subgrade 17
underdrains. 18
5.6 CROSS DRAINAGE 19
5.6.1 INTRODUCTION 20
A culvert is a closed conduit under a roadway or embankment used to maintain flow from a 21
natural channel or drainage ditch. A culvert should convey flow without causing damaging 22
backwater, excessive flow constriction, or excessive outlet velocities. 23
In addition to determining the design flows and corresponding hydraulic performance of a 24
particular culvert, other factors can affect the ultimate design of a culvert and should be 25
taken into consideration. These factors can include the economy of alternative pipe 26
materials and sizes, horizontal and vertical alignment, environmental concerns, and the 27
necessary culvert end treatments. 28
During a given storm event, a culvert may operate under inlet control, outlet control, or both. 29
Different variables and equations determine the culvert‘s capacity for each type of control. 30
5.6.2 CULVERT TYPES 31
Common culvert shapes generally available and used in highway applications are: 32
5 - 38 August 2010
1
2 Figure 5-4: Commonly-Used Culvert Shapes 3
5.6.3 FIELD DATA, PLANNING, AND LOCATION 4
The collection of field data for culvert hydraulics should be accomplished concurrently with 5
other data collection efforts for the project. The planning and location of a culvert should be 6
consistent with the floodplain encroachment requirements. Below is the list of minimum field 7
data required to complete an engineering analysis of a culvert installation: 8
Topographic map showing contours and the outline of the drainage area. 9
Description of the ground cover of the drainage area. 10
Streambed description and gradation at the proposed site. 11
Soils investigation. 12
Streambed alignment and profile extending twice the diameter at the proposed site. 13
The distance will vary based on the size of the culvert and the location. If the culvert 14
is 48 in., use two times the diameter for the distance in feet. For example, a 48-in. 15
culvert would require 48 x 2 = 96 ft. upstream and downstream, for a total of 192 ft., 16
plus the culvert length for the stream profile. 17
Cross sections of the stream width extending beyond the limits of the floodplain on 18
each side. 19
Proposed roadway profile and alignment in the vicinity of the culvert. 20
Proposed roadway cross section at the culvert. 21
Corrosion zone location, pH, and resistivity of the site. 22
Historical information at the site from maintenance or the locals. 23
5 - 39 August 2010
Any other unique features that can affect design, such as low-lying structures that 1
could be affected by excessive headwater or other considerations. 2
Inspect and document the physical condition of the existing culvert. 3
5.6.4 CULVERT FUNDAMENTALS 4
Flow Conditions ― A culvert barrel may flow full over all of its length or partly full. Full flow 5
in a culvert barrel is rare. Generally, at least part of the barrel flows partly full. A water 6
surface profile calculation is the only way to accurately determine how much of the barrel 7
flows full. 8
Full Flow ― The hydraulic condition in a culvert flowing full is called pressure flow. If the 9
cross-sectional area of the culvert in pressure flow is increased, the flow area will expand. 10
One condition that can create pressure flow in a culvert is the back pressure caused by a 11
high downstream water surface elevation. A high upstream water surface elevation may 12
also produce full flow. Regardless of the cause, the capacity of a culvert operating under 13
pressure flow is affected by upstream and downstream conditions and by the hydraulic 14
characteristics of the culvert. 15
Partly Full (Free Surface) Flow ― Free-surface flow or open-channel flow may be 16
categorized as sub-critical, critical, or super-critical. A determination of the appropriate flow 17
regimen is accomplished by evaluating the dimensionless number, Fr, called the Froude 18
Number. When Fr > 1.0, the flow is super-critical and is characterized as swift. When Fr < 19
1.0, the flow is sub-critical and characterized as smooth and tranquil. If Fr = 1.0, the flow is 20
said to be critical. 21
Types of Flow Control ― Inlet and outlet control are the two basic types of flow control. 22
The basis for the classification system is the location of the control section. The 23
characterization of pressure and the sub-critical and super-critical flow regimens play an 24
important role in determining the location of the control section, and thus the type of control. 25
The hydraulic capacity of a culvert depends on a different combination of factors for each 26
type of control. 27
Inlet Control ― Inlet control occurs when the culvert barrel is capable of conveying more 28
flow than the inlet will accept. The control section of a culvert operating under inlet control is 29
located just inside the entrance. Critical depth occurs at or near this location, and the flow 30
regimen immediately downstream is super-critical. Hydraulic characteristics downstream of 31
the inlet control section do not affect the culvert capacity. The upstream water surface 32
elevation and the inlet geometry represent the major flow controls. The inlet geometry 33
includes the barrel shape, the cross-sectional area, and the inlet edge. 34
Outlet Control ― Outlet control flow occurs when the culvert barrel is not capable of 35
conveying as much flow as the inlet opening will accept. The control section for outlet 36
control flow in a culvert is located at the barrel exit or further downstream. Either sub-critical 37
or pressure flow exists in the culvert barrel under these conditions. All of the geometric and 38
5 - 40 August 2010
hydraulic characteristics of the culvert play a role in determining its capacity. These 1
characteristics include all of the factors governing inlet control; the water surface elevation 2
at the outlet; and the slope, length, and hydraulic roughness of the culvert barrel. 3
Headwater ― Energy is required to force flow through a culvert. This energy takes the form 4
of an increased water surface elevation on the upstream side of the culvert. The depth of 5
the upstream water surface measured from the invert at the culvert entrance is generally 6
referred to as headwater depth. 7
A considerable volume of water may be ponded upstream of a culvert installation under 8
high fills or in areas with flat ground slopes. The ponding on the upstream of roadway fills is 9
discouraged for the general case unless the ponding is a pre-construction existing case and 10
the ponding has no detrimental effect on upstream structures and other property. 11
Allowable Headwater ― Circular culverts, box culverts, and pipe arches should be 12
designed so that the ratio of the headwater (HW) to diameter (D) during the design flow 13
event is less than or equal to 1.25 (HWi/D < 1.25). HWi/D ratios larger than 1.25 are 14
permitted, provided that existing site conditions such as available ponding areas on deeper 15
fills, warrant a larger ratio. The maximum allowable HWi/D ratios should not exceed 3.0. 16
The headwater that occurs during the 100-year flow event must also be investigated. Two 17
sets of criteria exist for the allowable headwater during the 100-year flow event, depending 18
on the type of roadway over the culvert: 19
1. If the culvert is under a main highway route that must be kept open during major 20
storm events, the culvert must be designed so that the 100-year flow event can be 21
passed without overtopping the roadway. 22
2. If the culvert is under a secondary roadway, it is recommended that the culvert be 23
designed so that there is no roadway overtopping during the 100-year flow event. 24
However, there may be situations where it is more cost-effective to design the 25
roadway embankment to withstand overtopping rather than to provide a structure or 26
group of structures capable of passing the design flow. 27
Bottomless culverts with footings should be designed so that 1 foot of debris clearance from 28
the water surface to the culvert crown is provided during the design flow event. Bottomless 29
culverts should also be designed so that the 100-year event can be passed without the 30
headwater depth exceeding the height of the culvert. Flow depths greater than the height 31
can cause potential scour problems near the footings. 32
Tailwater ― Tailwater is defined as the depth of water downstream of the culvert measured 33
from the outlet invert. It is an important factor in determining culvert capacity under outlet 34
control conditions. Tailwater may be caused by an obstruction in the downstream channel 35
or by the hydraulic resistance of the channel. In either case, backwater calculations from 36
the downstream control point are required to precisely define tailwater. When appropriate, 37
normal depth approximations may be used instead of backwater calculations. 38
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Outlet Velocity ― Because a culvert usually constricts the available channel area, flow 1
velocities in the culvert are likely to be higher than in the channel. These increased 2
velocities can cause streambed scour and bank erosion in the vicinity of the culvert outlet. 3
Energy dissipaters and outlet protection devices are oftentimes required to avoid scour at 4
the culvert outlet. All culverts must be designed for downstream channel stability. The 5
velocity of flow from the culvert outlet or energy dissipater/protection section should be 6
laminar, sub-critical, and in no case exceed the natural velocity of the existing channel. 7
Alignment and Grade ― It is recommended that culverts be placed on the same alignment 8
and grade as the natural streambed, especially on year-round streams. This tends to 9
maintain the natural drainage system and minimize downstream impacts. Locate and use 10
headwalls to minimize fill impacts within the existing stream ordinary high water level. 11
Allowable Velocities ― Culverts should be designed for the required life span (50 years 12
on main routes, 25 years on secondary roads), where corrosion and abrasion are the 13
primary cause of pipe failure. The type of culvert material, dimensions, and configurations 14
should be compatible with the velocity to reduce abrasion wear to acceptable levels, while 15
operating within acceptable hydraulic parameters with stable inlets and outlet conditions. 16
Minimum Spacing ― When multiple lines of pipe or pipe-arch greater than 48 in. in 17
diameter or span are used, they should be spaced so that the sides of the pipe are no 18
closer than one-half a diameter or 3 ft., whichever is less, so there is space for adequate 19
compaction of the fill material available. For diameters up to 48 in., the minimum distance 20
between the sides of the pipe should be no less than 2 ft. Utility lines may be closer. 21
5.6.5 ENGINEERING ANALYSIS 22
Collected field data should be used to perform an engineering analysis. The intent of the 23
engineering analysis is to ensure that the designer considers a number of issues, including 24
flow capacity requirements, foundation conditions, embankment construction, runoff 25
conditions, soil characteristics, stream characteristics, construction problems that may 26
occur, estimated cost, environmental concerns, and any other factors that may be involved 27
in and pertinent to the design. An additional analysis may be required in floodplain areas to 28
demonstrate that the culvert does not increase upstream levels over existing conditions. 29
Once the engineering analysis is completed, it will be part of the hydraulic report and should 30
include: 31
Culvert hydraulic and hydrology calculations. 32
Approved modeling software, such as HY-8, can also be used in lieu of hand 33
calculations. If the designers wish to use different software, prior approval is 34
required prior to submitting final designs. 35
Proposed roadway stationing of the culvert location. 36
Culvert and stream profile. 37
5 - 42 August 2010
Culvert length and size. The minimum diameter of culvert pipes under a main 1
roadway should be 18 in. Culvert pipes under roadway approaches should have a 2
minimum diameter of 12 in. 3
Culvert material (for culverts larger than 48 in., with appropriate n values). 4
Headwater depths, Water Surface Elevations (WSEL), and flow rates (Q) for the 5
design flow event (generally, the 25-year event and the 100-year flow event) should 6
appear on the plan sheets for future record. 7
Proposed roadway cross section and roadway profile demonstrating the maximum 8
and minimum height of fill over the culvert. 9
Appropriate end treatment. 10
Hydraulic features of downstream controls, tailwater, or backwater (storage) 11
conditions. 12
5.6.6 HYDRAULIC DESIGN OF CULVERTS 13
A complete theoretical analysis of the hydraulics of a particular culvert installation is time 14
consuming and complex. Flow conditions vary from culvert to culvert and can also vary over 15
time for any given culvert. The barrel of the culvert may flow full or partially full depending 16
on upstream and downstream conditions, barrel characteristics, and inlet geometry. The 17
design of all culverts should be done in accordance with FHWA publication HDS 5, 18
Hydraulic Design of Highway Culverts, No. FHWA-NHI-01-020, September 2001. 19
5.6.7 CULVERT END TREATMENTS 20
The type of end treatment used on a culvert depends on many interrelated and sometimes 21
conflicting considerations. The designer must evaluate safety, aesthetics, debris capacity, 22
hydraulic efficiency, scouring, and economics. Each end condition may serve to meet some 23
of these purposes, but none can satisfy all of these concerns. The designer must use good 24
judgment to arrive at a compromise as to which end treatment is most appropriate for a 25
specific site. 26
A number of different types of end treatments will be discussed in this section. The type of 27
end treatment chosen for a culvert should be specified in the contract plans for each 28
installation. 29
Projecting Ends 30
A projecting end is a treatment where the culvert is simply allowed to protrude out of the 31
embankment (see Figure 5-5). The primary advantage of this type of end treatment is that it 32
is the simplest and most economical of all treatments. Projecting ends also provide 33
excellent strength characteristics because the pipe consists of a complete ring structure out 34
to the culvert end. 35
5 - 43 August 2010
There are several disadvantages to projecting ends. For metal, the thin wall thickness does 1
not provide flow transition into or out of the culvert, significantly increasing head losses (the 2
opposite is true for concrete -- the thicker wall provides a more efficient transition). From an 3
aesthetic standpoint, projecting ends may not be desirable in areas exposed to public view. 4
They should only be used when the culvert is located in the bottom of a ravine or in rural 5
areas. Roadway safety considerations require that no projecting ends be allowed in the 6
designated clear zone. 7
Projecting ends are also susceptible to flotation when the inlet is submerged during high 8
flows. Flotation occurs when an air pocket forms near the projecting end, creating a 9
buoyant force that lifts the end of the culvert out of alignment. The air pocket can form when 10
debris plugs the culvert inlet or when significant turbulence occurs at the inlet as flow enters 11
the culvert. Flotation tends to become a problem when the diameter exceeds 6 ft. for metal 12
pipe and 2 ft. for thermoplastic pipe. It is recommended that pipes which exceed those 13
diameters be installed with a beveled end and a concrete headwall or slope collar. Concrete 14
pipe will not experience buoyancy problems and can be projected in any diameter. 15
However, because concrete pipe is fabricated in relatively short 6-foot to 12-foot sections, 16
the sections are susceptible to erosion and corresponding separation at the joint. 17
18 Figure 5-5: Projecting End 19
Beveled End Sections 20
A beveled end treatment consists of cutting the end of the culvert at an angle to match the 21
embankment slope surrounding the culvert. A beveled end provides a more hydraulically 22
efficient opening than a projecting end, is relatively cost-effective, and is generally 23
considered to be aesthetically acceptable. Beveled ends should be considered for culverts 24
approximately 6 ft. in diameter and less. If culverts larger than approximately 6 ft. in 25
diameter are beveled but not reinforced with a headwall or slope collar, the structural 26
integrity of the culvert can be compromised, and failure can occur. The standard beveled 27
end section should not be used on culverts placed on a skew of more than 30 degrees from 28
perpendicular to the centerline of the highway. Cutting the ends of a corrugated metal 29
culvert structure to an extreme skew or bevel to conform to the embankment slope destroys 30
the ability of the end portion of the structure to act as a ring in compression. Headwalls, 31
riprap slopes, slope paving, or stiffening of the pipe may be required to stabilize these ends. 32
In these cases, special end treatments should be provided, if needed. 33
5 - 44 August 2010
1 2
Figure 5-6: Beveled End Section 3
Flared End Sections 4
A metal flared end section is a manufactured culvert end that provides a simple transition 5
from culvert to streambed. Flared end sections allow flow to smoothly constrict into a culvert 6
entrance and then spread out at the culvert exit as flow is discharged into the natural 7
stream or watercourse. Flared ends are generally considered aesthetically acceptable 8
because they serve to blend the culvert end into the finished embankment slope. 9
Flared end sections are typically used only on circular pipe or pipe arches. Flared ends are 10
generally constructed out of steel and aluminum and should match the existing culvert 11
material, if possible. However, either type of end section can be attached to concrete or 12
thermoplastic pipe, and the contractor should be given the option of furnishing either steel 13
or aluminum flared end sections for those materials. 14
A flared end section is usually the most feasible option in smaller pipe sizes and should be 15
considered for use on culverts up to 48 in. in diameter. For diameters larger than 48 in., end 16
treatments such as concrete headwalls tend to become more economically viable than the 17
flared end sections. 18
The undesirable safety properties of flared end sections generally prohibit their use in the 19
clear zone for all but the smallest diameters. A flared end section is made of light gage 20
metal, and because of the overall width of the structure, it is not possible to modify it with 21
safety bars. When the culvert end is within the clear zone and safety is a consideration, the 22
designer must use a tapered end section with safety bars. The tapered end section is 23
designed to match the embankment slope and allow an errant vehicle to negotiate the 24
culvert opening in a safe manner. 25
26 Figure 5-7: Flared End Section 27
5 - 45 August 2010
Slope Collars 1
A slope collar is a concrete frame poured around a beveled culvert end. It provides 2
structural support to the culvert and eliminates the tendency for buoyancy. A slope collar is 3
generally considered to be an economically feasible end treatment for metal culverts that 4
range in size from 6 to 10 ft. Generally, metal culverts smaller than 6 ft. do not need the 5
structural support provided by a slope collar. Slope collars should be used on thermoplastic 6
culverts larger than 2 ft. When the culvert is within the clear zone, the collar design can be 7
modified by adding safety bars. The designer is cautioned not to use safety bars on a 8
culvert where debris may cause plugging of the culvert entrance, even though the safety 9
bars may have been designed to be removed for cleaning purposes. When the stream is 10
known to carry debris, the designer should provide an alternate solution to safety bars, such 11
as increasing the culvert size or providing guardrail protection around the culvert end. 12
Slope collars for culverts larger than 10 ft. tend to lose cost-effectiveness due to the large 13
volume of material and forming cost required for this type of end treatment. Instead, a 14
headwall is recommended for culverts larger than 10 ft. 15
16 Figure 5-8: Slope Collar 17
Headwalls, Wingwalls, and Aprons 18
Headwalls with or without wingwalls and aprons are intended for use where site constraints 19
restrict the fill slope and fill retention is needed, or where flow transitions are needed to 20
avoid erosion of the channel slopes, and for box culverts. Their purpose is to retain and 21
protect the embankment and provide a smooth transition between the culvert and the 22
channel. Normally, they will consist of vertical headwall, flared vertical wingwalls, a full or 23
partial apron, and bottom and side cutoff walls (to prevent piping and undercutting). The 24
wingwall and apron will provide a smooth transition for the flow as it spreads to the natural 25
channel. Headwalls may be precast or cast-in-place reinforced concrete structures, 26
designed to be stable for the soil loadings particular to the site. 27
5 - 46 August 2010
1 Figure 5-9: Headwall with Wingwalls for Circular Pipe 2
Improved Inlets 3
In conditions of inlet control, techniques available to balance the inlet capacity include 4
beveled inlet edges, the side tapering of inlet, and the slope-tapering of inlet. When the 5
head losses in a culvert are critical, the designer may consider using a hydraulically-6
improved inlet. These inlets provide side transitions as well as top and bottom transitions 7
that have been carefully designed to maximize the culvert capacity with the minimum 8
amount of headwater; however, the design and form construction costs can become quite 9
high for hydraulically-improved inlets. For this reason, their use is not encouraged in routine 10
culvert design. It is usually less expensive to simply increase the culvert diameter by one or 11
two sizes to achieve the same or greater benefit. 12
Inlet Bevel 13
Beveled edges are usually a standard component of the inlet ends of box culverts and 14
headwall structures. A bevel is similar to a chamfer, except that a chamfer is smaller and is 15
generally used to prevent damage to sharp concrete edges during construction. The 16
entrance loss coefficient can be reduced from 0.7 for a square edge to 0.2 for beveled 17
edges. It should be noted that the socket end of concrete pipe is comparable to a bevel in 18
reducing the entrance loss coefficient and thus increasing the inlet capacity. 19
Side-Tapered Inlet 20
Side-tapered inlets provide an enlarged culvert entrance with transitions to original barrel 21
dimensions. The inlet face has the same height as the barrel, and its top and bottom are 22
extensions of the top and bottom of the barrel. The intersection of the side wall tapers and 23
barrel is called the throat section. The use of a side-tapered improvement is maximized by 24
designing it so that the capacity is controlled by the throat. 25
Slope-Tapered Inlets 26
Slope-tapered inlets provide a steeper slope at the entrance than occurs throughout the 27
remaining length of a culvert. Providing this steeper slope allows the head on the throat 28
section to be increased to make additional fall available. Depending upon the fall, inlet 29
5 - 47 August 2010
capacities can be increased 100 percent or greater above conventional culverts with square 1
edges. 2
5.6.8 CONSTRUCTION AND MAINTENANCE CONSIDERATION 3
The design of a culvert should be consistent with standard construction and specifications. 4
If special provisions are warranted in order to properly construct or maintain the proposed 5
facility, this information should be specified in the design documentation. The designer must 6
also consider maintenance of traffic requirements because cost and disruption of the 7
installation may be the major concern for some locations. 8
In all cases, the culverts should be designed to have self-cleaning velocities, and both ends 9
should have some type of erosion protection. The material selected should have a 50-year 10
or longer life span estimate, because most roads can be resurfaced a number of times to 11
increase their life without the reconstruction of subgrade. 12
The use of multiple culvert openings is discouraged except in situations where very little 13
vertical distance is available from the roadway to the culvert invert. Alternatives to multiple 14
culvert installations are low-profile arches and three-sided box structures that provide 15
significant horizontal span lengths while minimizing the vertical rise. 16
Where possible, maintenance access should be provided at the upstream end of cross 17
culverts. For shallow culverts, this can be a widened shoulder pull-out on which to park a 18
vehicle. For culverts in deeper fills, provide a separate maintenance access road from off 19
the embankment. Access roads should be a minimum of 12 ft. wide and should not exceed 20
a 15 percent slope. The maintenance road should be paved to match the anticipated 21
maintenance vehicle loadings, but should never be less than a 6-in. thick layer of crushed 22
gravel over a geotextile filter fabric. 23
Safety fencing should be provided along the tops of all culvert headwalls with a vertical face 24
of 6 ft. or more. 25
5.6.9 ENERGY DISSIPATERS 26
When the outlet velocities of a culvert are excessive for the site conditions, the designer 27
must consider erosion protection. The protection can be fairly simple inlet and outlet 28
channel stabilization methods, such as riprap slope and bottom protection, splash pads, 29
and vegetated slope protection measures. Or it may require the installation of energy 30
dissipaters. Dissipaters are hydraulic control structures to reduce velocities to within 31
tolerable limits. Culvert erosion protection and energy dissipaters should be designed in 32
accordance with FHWA publication HEC 14, Hydraulic Design of Energy Dissipaters for 33
Culverts and Channels, No. FHWA-NHI-06-086, July 2006. Typical protected outlets and 34
energy dissipaters include: 35
36
37
5 - 48 August 2010
Riprap Protected Outlets 1
A common mitigation measure for small culverts is to provide at least minimum protection 2
using riprap aprons. The initial protection against channel erosion should be sufficient to 3
provide some assurance that extensive damage could not result from one runoff event. 4
These aprons do not dissipate significant energy except through increased roughness for a 5
short distance. However, they do serve to spread the flow helping to transition to the natural 6
drainage way or to sheet flow where no natural drainage way exists. The configuration and 7
riprap sizing is defined in Section 10.2 of HEC 14. 8
For larger culverts, the designer should consider estimating the size and riprapping the 9
scour hole using the procedures in Chapter 5 of HEC 14, or using a riprap basin in 10
accordance with Section 10.1 of HEC 14. 11
Where suitably-sized riprap rock is unavailable, gabion baskets and mattresses are 12
alternatives to using loose rock. The gabions are wire mesh baskets that are filled with 13
smaller-sized rock. They can provide the same level of erosion protection using smaller-14
sized rock, usually easier to handle and more readily available than riprap materials. See 15
Chapter 6 of FHWA publication HEC 11, Design of Riprap Revetment, No. FHWA-IP-89-16
016, March 1989 for guidelines pertaining to design of wire-encased rock. 17
Splash Pads 18
Concrete splash pads are constructed in the field at the outlet end of smaller culverts and 19
storm drain discharges to prevent erosion. Splash pads should be a minimum of three times 20
the diameter in width and four times the diameter in length, as shown in Figure 5-10. 21
22
23 Figure 5-10: Splash Pad Details 24
25
26
5 - 49 August 2010
Other Energy Dissipating Structures 1
Other structures include impact basins, stilling basins/wells, and drop structures. These 2
structures may consist of baffles, posts, or other means of creating roughness and 3
hydraulic jumps to dissipate excessive velocity. See FHWA publication HEC 14 for the 4
selection and design criteria. 5
Energy dissipaters have a reputation for collecting debris on the baffles, so the designer 6
should consider this possibility when choosing a dissipater design. In areas of high debris, 7
the dissipater should be a self-clearing type and easily accessible to maintenance crews. 8
5.6.10 CULVERT DEBRIS 9
Debris problems can cause even an adequately-designed culvert to experience hydraulic 10
capacity problems. Debris may consist of anything from limbs and sticks, or orchard 11
pruning, to logs and trees. Silt, sand, gravel, and boulders can also be classified as debris. 12
The culvert site is a natural place for these materials to settle and accumulate. No method 13
is available for accurately predicting debris problems. Examining the maintenance history of 14
each site is the most reliable way of determining potential problems. Sometimes, upsizing a 15
culvert is necessary to enable it to pass debris more effectively. Upsizing may also allow a 16
culvert to be more easily cleaned. 17
Methods to counter debris at culverts are discussed in Section 5.2 of FHWA publication 18
HEC 9, Debris Control Structures, Evaluation and Countermeasures, Third Edition, No. 19
FHWA-IF-04-016, October 2005. In all cases, culverts should be designed to have 20
maintenance access for debris removal at the inlets. 21
22
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6 - 1 August 2010
6 PERMANENT BEST MANAGEMENT PRACTICES 1
6.1 INTRODUCTION 2
This chapter provides designers with specific guidelines for the selection, design, and 3
application of permanent stormwater Best Management Practices (BMPs). 4
Stormwater BMPs are the physical, structural, and managerial practices that, when used 5
alone or in combination, prevent or reduce the detrimental impacts of stormwater, such 6
as the pollution of water, degradation of channels, damage to structures, and flooding. 7
The BMP principles adopted in this Transportation Stormwater Drainage Manual (TSDM) 8
are widely-accepted nationwide. However, the BMPs presented have been selected as 9
being more applicable to the linear nature of transportation projects, with details modified 10
to better fit with roadway features and limited right-of-ways. 11
For each BMP, this chapter presents a selection process along with design 12
considerations. This chapter also presents ways to combine or enhance the different 13
types of facilities to maximize their efficiency or to better fit within the project site. 14
The BMP descriptions include general recommendations regarding the conditions under 15
which a BMP applies, as well as the advantages and disadvantages of that BMP. It is 16
recommended that designers take an iterative approach to selecting BMPs based on 17
site-specific criteria. This entails being flexible and somewhat creative when determining 18
a final stormwater management solution that works best in each situation. It is essential 19
that care be taken in the selection and site application of the various BMPs, such that 20
maximum benefit is obtained. 21
6.2 TYPES AND FUNCTIONS OF BMPs 22
Permanent BMPs are generally categorized by design pollution prevention, runoff 23
treatment, and flow control as shown in Table 6-1. The BMP selection process, design 24
applications, and details are described in the following sections. 25
26
BMP Description
Design Pollution Prevention BMPs
Consideration of downstream effects related to potentially increased flow velocities; preservation of existing vegetation; concentrated flow conveyance systems; and slope/surface protection systems.
Quality Treatment BMPs
Permanent runoff quality treatment devices and facilities for the removal of pollutants prior to discharge back to the natural or existing watercourse.
6 - 2 August 2010
BMP Description
Flow Control BMPs
Detention or infiltration type facilities for purposes of minimizing downstream channel erosion and overbank flooding, and/or for groundwater recharge.
Table 6-1: Permanent BMP Categories 1
The stormwater management methods in this chapter have been categorized in order of 2
preferred use and grouped according to similar composition and function. Each BMP has 3
an associated number to distinguish it from other BMPs with similar names. The 4
numbering convention represents the following classifications: 5
6
RT.XX – Runoff Treatment BMPs
FC.XX – Flow Control BMPs
IN.XX – Infiltration BMPs
Table 6-2: Permanent BMP Numbering Conventions 7
6.2.1 DESIGN POLLUTION PREVENTION BMPS 8
A main consideration in project design should be the prevention of pollutants from 9
entering stormwater by eliminating the source of pollution or by preventing the contact of 10
pollutants with rainfall and runoff. The three design objectives are: 11
1. Maximize vegetated surfaces: Vegetated surfaces prevent erosion, help the 12
pollutant removal process, and promote infiltration, which reduces runoff. 13
2. Prevent downstream erosion: Apply designs to minimize impacts from any 14
downstream increases in velocity. 15
3. Stabilize soil areas: Areas prone to erosion will be stabilized through the use of 16
flow control and grading/cover improvements. 17
These design objectives should be applied to the entire project, both existing and new 18
project areas. The typical design pollution prevention BMPs that accomplish this are: 19
Preservation of vegetation 20
o Design of permanent facilities to minimize ground disturbance areas 21
o Construction staging to avoid stands of trees and shrubs 22
Consideration of downstream effects related to potentially increased flow 23
velocities 24
o Making modifications to channel lining materials (both natural and man-25
made), including vegetation, geotextile mats, rock, and riprap 26
6 - 3 August 2010
o Using energy-dissipation devices at culvert outlets 1
o Smoothing the transition between culvert outlets, headwalls, wingwalls, and 2
channels to reduce turbulence and scour 3
o Incorporating detention and infiltration into designs to reduce peak discharge 4
rates 5
Concentrated flow conveyance systems to avoid gullying and erosion 6
o Ditches, berms, dikes, and swales 7
o Oversize drains 8
o Flared-end culvert sections 9
o Protective outlet treatments 10
Slope/surface protection systems 11
o Vegetated surfacing 12
o Surface roughening, terracing, stepping, or rounding 13
o Hard surfacing 14
6.2.2 QUALITY TREATMENT BMPS 15
Runoff treatment BMPs that remove pollutants from runoff use a variety of mechanisms 16
including sedimentation, filtration, plant uptake, ion exchange, adsorption, precipitation, 17
and bacterial decomposition. The following overview provides information on the most 18
commonly used runoff treatment BMPs: 19
Infiltration BMPs: Infiltration BMPs are the preferred method for both runoff 20
treatment and flow control. Infiltration treatment has the highest pollutant removal 21
efficiency of the listed BMPs. Treatment is achieved through settling, biological 22
action, and filtration. The advantage of this type of BMP is that it also provides 23
recharge of the ground water. It is a primary BMP for quality treatment in the 24
northern Guam area, where the goal is to infiltrate as much of the project runoff 25
as possible. All infiltration facilities must be preceded by a presettling basin for 26
removing most of the sediment particles that would otherwise reduce the 27
infiltrative capacity of the facility. The infiltration BMPs include: 28
29
IN.01 - Bioinfiltration Pond
IN.02 - Infiltration Pond
IN.03- Infiltration Trench
IN.04 - Infiltration Vault
IN.05 -Drywell
IN.06- Permeable Pavements
30
6 - 4 August 2010
Dispersion BMPs: Perhaps the single most promising and effective approach to 1
mitigating the effects of highway runoff is to look for opportunities to use the 2
existing natural areas for pollutant removal. Natural dispersion requires that 3
runoff sheet flows off of the pavement where it flows through the natural 4
vegetation without being channelized. Pollutant removal typically occurs through 5
a combined process of vegetative filtration and shallow surface infiltration. 6
A good portion of the existing roadway runoff in Guam is already treated by 7
natural dispersion. If this condition can be maintained within the project limits for 8
new projects, natural dispersion can be very cost effective while helping to 9
preserve the natural environmental functions, including flow attenuation. 10
Engineered dispersion techniques use the same removal processes as natural 11
dispersion, but use a constructed conveyance system to direct runoff to the sheet 12
flow dispersion area. Dispersion BMPs are: 13
14
RT.01 - Natural Dispersion
RT.02 - Engineered Dispersion
Biofiltration BMPs: Runoff treatment to remove pollutants can be best 15
accomplished before concentrating the flow. Biofiltration functions slow the runoff 16
velocities, filter out sediment and other pollutants, and provide some infiltration 17
into the underlying soils. Although limited to fairly flat terrain, vegetated filter 18
strips, continuous inflow biofiltration swales, and media filter drains provide direct 19
treatment of the sheet flow off of the pavement. Biofiltration swale is similar in 20
function, but able to treat concentrated runoff flows. 21
All of the following BMPs are a cost-effective treatment option, suitable for use 22
within the typical limited width right-of-way. 23
24
RT.03 -Vegetated Filter Strip RT.05 - Wet Biofiltration Swale
RT.04 - Biofiltration Swale
RT.06 - Continuous Inflow Biofiltration Swale
RT.07 -Media Filter Drain
Wet Pool BMPs: Wet ponds are constructed basins containing a permanent pool 25
of water throughout the wet season. Wet ponds function by settling suspended 26
solids, where the biological action of plants and bacteria provides some 27
additional treatment. Wet ponds are usually constructed using multiple cells, 28
where coarser sediments become trapped in the first cell or forebay. 29
Wet pond designs can also provide flow control by adding detention volume (live 30
storage) above the dead storage. Wet ponds can be designed for the treatment 31
of conventional pollutants, as well as enhanced pollutant removal using 32
6 - 5 August 2010
constructed stormwater treatment wetlands for removal of nutrients and 1
dissolved metals. Designers of constructed stormwater treatment wetlands 2
should consider the landscape context as groundwater, soils, and surrounding 3
vegetation must be compatible for long-term maintenance of the wetland plants. 4
Applicable wet pool BMPs are: 5
6
RT.08 - Wet Pond
RT.10 - Constructed Stormwater Treatment Wetland
RT.09 – Combined Wet/Detention Pond
RT.11 - Combined Stormwater Treatment Wetland/Detention Pond
Phosphorous Control BMPs: Where runoff discharge is to a Guam Environmental 7
Protection Agency (GEPA)-designated resource protection area, enhanced 8
phosphorous removal may be required. BMPs that are applicable to this 9
requirement are listed below. The wet pond BMP size is increased 1.5 times the 10
basic wet pond treatment volume for phosphorous control. 11
12
RT.08 - Wet Pond (Large)
RT.07 - Media Filter Drain
There are additional BMPs that are generally accepted for stormwater quality 13
treatment; however they have high maintenance requirements. Some of these 14
are listed below. The use of any of the following BMPs will require pre-approval 15
by the Department of Public Works (DPW): 16
17
Sand Filters
Underground Wet Vaults and Sedimentation Chambers
Proprietary Sediment Concentrators and Pollutant-Removal Systems
Proprietary Filtration Systems
Coalescing Plate Oil/Water Separators
6.2.3 FLOW CONTROL BMPS 18
Stormwater flow control BMPs are designed to control the flow rate or the amount of 19
runoff leaving a site after development. The primary mechanisms used to manage flow 20
control include dispersion, infiltration, and detention. Increased flow rates can cause 21
downstream damage due to flooding, erosion, and scour, as well as degradation of 22
water quality and in-stream habitat because of channel and stream bank erosion. The 23
following provides an overview of the most commonly used flow control BMPs for 24
transportation type applications: 25
6 - 6 August 2010
Infiltration BMPs: Infiltration is the preferred BMP for both stormwater treatment 1
and flow control. Infiltration BMPs will be almost exclusively required for use in 2
the northern Guam area due to aquifer recharge requirements. However, 3
infiltration systems are practicable only in areas where groundwater tables are 4
sufficiently below the bottom of the facility and in highly permeable soil 5
conditions. Also, all infiltration systems require some type of pre-treatment 6
measures, such as a swale or sediment basin to protect the infiltration facility 7
from sedimentation clogging. 8
Subsurface infiltration systems should be considered only when room is 9
inadequate to construct an infiltration basin. Underground systems are difficult to 10
maintain and verify whether they are functioning properly. 11
12
IN.01 - Bioinfiltration Pond
IN.02 - Infiltration Pond
IN.03 - Infiltration Trench
IN.04 - Infiltration Vault
IN.05 - Drywell
Dispersion BMPs: See Section 6.2.2 above for an overview of dispersion BMPs. 13
14
RT.01 - Natural Dispersion
RT.02 - Engineered Dispersion
Detention BMPs: Detention facilities generally take the form of either a pond or 15
an underground vault or tank. They operate by providing a volume of live storage 16
with an outlet control structure designed to release flow at a reduced rate over 17
time. Ponds can be configured as dry ponds to control flow only, or combined 18
with a wet pond or created wetland to also provide runoff treatment within the 19
same footprint. 20
21
FC.01 – Detention Pond RT.09 – Combined Wet/Detention Pond
RT.11 - Combined Stormwater Treatment Wetland/Detention Pond
22
6.2.4 BMP SELECTION PROCESS 23
The appropriate selection of BMPs requires designers to have an understanding of the 24
process used to identify water quality requirements and pollutants of concern for specific 25
water bodies. 26
The term ―permanent BMP‖ refers to permanent physical controls that control, prevent, 27
remove, or reduce pollution, and minimize potential impacts on receiving waters. 28
6 - 7 August 2010
Accordingly, the BMPs are usually selected during the design phase and will be 1
operational post-construction. 2
Selection of the BMP that best fits a specific application should be generally approached 3
using the following main steps. 4
Step 1. Determine the applicable Minimum Requirements. 5
Review the procedures in Chapter 3 of this TSDM. Even if no quality treatment or flow 6
control BMPs are required, the designer should still proceed with Step 2. 7
Step 2. Select design pollution prevention BMPs. 8
See Figure 6-1 for the selection of design pollution prevention BMPs. 9
Step 2 a. Select flow control BMPs. 10
Based on the Minimum Requirements in Step 1, use Figure 6-2 for selection of flow 11
control BMPs. 12
Step 3. Select runoff treatment BMPs. 13
Based on the Minimum Requirements in Step 1, use Figure 6-3 for selection of runoff 14
treatment BMPs. 15
6 - 8 August 2010
1 Figure 6-1: Flow Chart – Selection of Design Pollution Prevention BMP 2
No
Check the applicability of Minimum Requirements (Section 3.3.1). Is the project exempt or below the minimum disturbance thresholds?
No further action needed.
Is the existing vegetation preserved to the maximum extent possible?
STABILIZE THE REMAINING DISTURBED AREA.
PERMANENT SEEDING AND PLANTING.
Yes
Yes
Begin selection of design pollution prevention BMPs.
Will the existing project increase the velocity of downstream flows?
Will the project create new slopes or modify existing slopes?
ACCESS DOWNSTREAM EFFECTS AND CONSIDER
ENERGY DISSIPATION AT OUTLETS
MODIFICATION TO CHANNEL LINING MATERIALS
SMOOTH CHANNEL TRANSITIONS
PROVIDE FLOW CONTROL
Will runoff channelize?
Yes MINIMIZE DISTURBANCE, STABILIZE SLOPES, AND
CONSIDER:
SLOPE SURFACE PROTECTION
PRESERVE EXISTING VEGETATION
CONCENTRATED FLOW CONVEYANCE SYSTEM
No
No
MINIMIZE GULLYING AND SCOUR AND CONSIDER:
CONCENTRATED FLOW CONVEYANCE SYSTEM (DITCHES, BERMS, DIKES, SWALES, AND OVERSIDE DRAINS)
DISPERSION AND INFILTRATION
Do cross drains exist?
Yes
Yes
MINIMIZE SCOUR AND EROSION AND CONSIDER:
CONCENTRATED FLOW CONVEYANCE SYSTEM (FLARED CULVERT END SECTIONS, OUTLET PROTECTION/ ENERGY-DISSIPATION DEVICES)
Yes
Document the decision for BMP selection.
Yes
No
No
6 - 9 August 2010
1 Figure 6-2: Flow Chart - Selection of Flow Control BMP 2
3
4
Yes
Yes
No Check the applicability of the Minimum Requirements. Does the project require flow control?
Meet other Minimum Requirements and assure that project stormwater is otherwise provided for.
Will dispersion or infiltration satisfy the flow control requirements?
Yes
Begin the selection of flow control BMPs.
Determine whether a combined runoff treatment and flow control BMP is possible. Will a combination runoff treatment and flow control BMP be used?
Choose a Detention BMP to satisfy flow control requirements.
Yes CHOOSE BMP:
COMBINED WET/ DETENTION POND
COMBINED CREATED WETLAND/DETENTION
POND
No
Document the decision for BMP selection.
SEE BMP DESIGN CRITERIA FOR INFILTRATION and DISPERSION
NATURAL DISPERSION
ENGINEERED DISPERSION
BIOINFILTRATION POND
INFILTRATION POND
INFILTRATION TRENCH
INFILTRATION VAULT
DRYWELL
CHOOSE DETENTION BMP: DETENTION POND
No
6 - 10 August 2010
1 Figure 6-3: Flow Chart – Selection of Runoff Treatment BMP 2
Yes
No
Yes Yes
No
Yes
Check the applicability of the Minimum Requirements. Does the project require runoff treatment?
Meet other Minimum Requirements and assure that project stormwater is otherwise provided for.
Was dispersion chosen for flow control?
No
Was infiltration chosen for flow control?
Determine receiving waters and pollutants of concern.
APPLY PRE-TREATMENT
Vegetated filter strips
Biofiltration swales
Wet biofiltration swales
Sedimentation basin
No
Yes
APPLY INFILTRATION
Infiltration basin
Infiltration trench
Infiltration vault
Drywell
Permeable Pavement
Does the soil have characteristics that would allow infiltration?
Check if phosphorous control is required.
APPLY PHOSPHOROUS CONTROL
Wet pond (large)
Media filter drain
Was a combined flow control and treatment facility chosen?
APPLY COMBINED FACILITY
Constructed wetland/ detention pond
Wet/detention pond
No
No
APPLY TREATMENT BMP
Vegetated filter strips
Biofiltration swales
Wet biofiltration swales
Wet pond
Media filter drain
Constructed wetland
Document decision for BMP selection.
APPLY DISPERSION
Natural dispersion
Engineered dispersion
Yes
Will soil amendment allow infiltration?
No Yes
6 - 11 August 2010
6.3 DESIGN POLLUTION PREVENTION BMPs 1
6.3.1 CONSIDERATION OF DOWNSTREAM EFFECTS RELATED TO 2
POTENTIALLY INCREASED FLOW VELOCITIES 3
Changes in the velocity or volume of runoff, the sediment load, or other hydraulic 4
changes from stream encroachments, crossings, or realignment may affect downstream 5
channel stability. 6
The designer will evaluate the effects on downstream channel stability and the 7
applicability of the mitigation measures described under the implementation for this 8
BMP. 9
Appropriate Applications 10
During the design of both new and reconstructed facilities, the designer may include new 11
road surfaces or additional surface paving to enhance the operational safety and 12
functionality of the facility. The designer must also consider the effect of collecting and 13
concentrating flows in roadside ditches and storm drain systems or the effect of re-14
directing flows to treatment BMPs. 15
Diversions or overflows from large storm events in these instances may create 16
concentrated discharges in areas that have not historically received these flows. 17
Implementation 18
If these changes result in an increased potential for downstream effects in channels, the 19
designer should consider the following: 20
Making modifications to channel lining materials (both natural and man-made), 21
including vegetation, geotextile mats, rock, and riprap. The primary goal will be to 22
use vegetative stabilization techniques whenever the conditions allow, such as 23
use of ―tree stakes‖ along toes and sides of channel banks, and/or provide 24
vegetation plantings to stabilize stream banks and overflow areas. 25
Using energy-dissipation devices at outlets. This includes such items as rock 26
pads, gabion pads, and constructed structures at outlets of ditches and pipes. 27
More natural alternatives include the discharge to spreader swales or spreader 28
ditches that sheet flow over the bank to the stream channel. 29
Smoothing the transition between culvert and ditch outlets to the natural 30
waterway using headwalls, wingwalls, and riprap/gabion protection to reduce 31
turbulence and scour. 32
Modifying the existing waterway to a more stable hydraulic design. 33
Incorporating detention and infiltration into designs to reduce peak discharge 34
rates. Typical applications are the inclusion of low areas in the terrain into the 35
conveyance system, over-sizing of conveyance system components, or providing 36
6 - 12 August 2010
specific detention ponds. The effectiveness of these features should be 1
determined by using a hydraulic routing computer model of the overall 2
conveyance system and matching the conveyance discharge hydrograph with 3
the stream hydrograph. A downstream analysis may be needed if the stream 4
flows are increased. Refer to Section 3.2.6.1 for more details. 5
The designer should implement the appropriate measures to ensure that runoff from 6
stormwater facilities will not increase downstream erosion. 7
6.3.2 PRESERVATION OF EXISTING VEGETATION 8
Description 9
The preservation of existing vegetation involves the identification and protection of 10
desirable vegetation. In particular, identification and protection of those locations which 11
provide specific runoff filtration and soil stabilization benefits. 12
Appropriate Applications 13
The designer should preserve existing vegetation at areas on a site where no 14
construction activity is planned or where it will occur at a later date. 15
Implementation 16
The following general steps should be taken to preserve existing vegetation: 17
Designers should review and include vegetation types/features on the 18
topographic surveys. Use the topographic surveys to prepare designs that 19
help to minimize the specific runoff benefit of vegetation removal. 20
Designers should identify and delineate in the contract documents all 21
vegetation to be retained. 22
The contractor should delineate the areas to be preserved in the field prior to 23
the start of soil-disturbing activities. 24
The contractor should minimize disturbed areas by locating temporary 25
construction access, work areas, and staging areas to avoid stands of trees 26
and shrubs, and follow existing contours to reduce cutting and filling. 27
When removing vegetation, the designer and contractor should consider 28
impacts (increased exposure or wind damage) to the adjacent vegetation that 29
will be preserved. 30
6.3.3 CONCENTRATED FLOW CONVEYANCE SYSTEMS 31
Concentrated flow conveyance systems consist of permanent design measures that are 32
used alone or in combination to intercept and divert surface flows, and to convey and 33
discharge concentrated flows with minimum soil erosion. 34
35
6 - 13 August 2010
Ditches, Berms, Dikes, and Swales 1
Description 2
These are permanent devices typically used to intercept and direct surface runoff to 3
an over-side (or slope) drain or stabilized watercourse. 4
Appropriate Applications 5
Ditches, berms, dikes, and swales are typically implemented: 6
At the top of slopes to divert run-on from adjacent slopes and areas 7
At bottom and mid-slope locations to intercept sheet flow and convey 8
concentrated flows 9
At other locations to convey runoff to over-side drains, stabilized 10
watercourses, and stormwater drainage system inlets (catch basins), pipes, 11
and channels 12
To intercept runoff from paved surfaces 13
Along roadways and facilities subject to flooding 14
Implementation 15
The hydraulic designs must be in accordance with Chapter 5 of this TSDM. Select 16
design storm frequencies based on the risk evaluation of erosion, overtopping, and 17
flow backups. Conveyances must be lined when velocities exceed allowable limits for 18
bare soil (see Chapter 5). Liners can consist of vegetative types, rock riprap, gabion 19
mats, and engineering erosion control fabric. 20
Consider outlet protection where localized scour is anticipated. Examine the site for 21
run-on from off-site sources and try to divert the flows around the work area. Review 22
the order-of-work provisions to install and use permanent dikes, swales, and ditches 23
early in the construction process. 24
Over-side Drains 25
Description 26
Over-side drains are conveyance systems used to protect slopes against erosion. 27
Over-side drains are usually piped drains, flumes, or paved spillways that protect 28
slopes against erosion by collecting surface runoff from the roadbed, the tops of cuts, 29
and benches (in cut or fill slopes), conveying it down the slope to a stabilized outlet. 30
Appropriate Applications 31
Over-side drains are typically used at sites where slopes may be eroded by gully flow 32
action. 33
34
6 - 14 August 2010
Implementation 1
The hydraulic design must be in accordance with Chapter 5 of this TSDM. 2
Pipe drains are storm sewer pipes adaptable to any slope. They are 3
recommended where side slopes are 1:4 (V:H) or steeper. Design and 4
installation will require additional energy dissipation and pipe stability 5
measures for high-velocity situations. 6
Pipe and flume down drains are surface-mounted pipes or cut pipe sections. 7
They are best adapted for low flow rates on slopes that are 1:2 (V:H) or 8
flatter. Pipe and flume down drains should only be used for temporary 9
applications, and must be securely anchored to the slope. 10
Paved spillways are recommended on side slopes flatter than 1:4 (V:H). On 11
steeper slopes, pipe drains should be used. 12
Provide over-side drains every 300 ft. to 500 ft. from benches in cut-and-fill 13
slopes. 14
Flared Culvert End Sections 15
Description 16
These are devices typically placed at inlets and outlets of pipes and channels to 17
improve the hydraulic operation, retain the embankment near pipe conveyances, and 18
help prevent scour and minimize erosion at these inlets and outlets. 19
Appropriate Applications 20
Use flared culvert end sections at outlets of storm drains, for over-side drains, and on 21
both the inlet and outlet of culverts. 22
Implementation 23
The design must be in accordance with Chapter 5 of this TSDM. Alternatives to 24
manufactured flared end sections are riprap pads, concrete slope collars, and 25
headwalls. 26
Outlet Protection/Velocity Dissipation Devices 27
Description 28
These devices are typically placed at pipe outlets to prevent scour and reduce the 29
outlet velocity and/or energy of exiting stormwater flows. They function to transition 30
from the ditch or pipe flow section to the existing waterway section. The purpose is to 31
protect the existing soil from erosion, while helping to dissipate flow energy and 32
transitioning the flow to a wider, flatter, and less velocity condition. Depending on the 33
flow energy, the outlet protection can consist of spreader swales or ditches, rock 34
6 - 15 August 2010
pads, riprap sections, flared-end sections, concrete collars, concrete pads, 1
headwalls, headwalls with wingwalls and aprons, and energy dissipater structures. 2
Appropriate Applications 3
These devices are typically used at the outlets of ditches, pipes, drains, culverts, 4
slope drains, diversion ditches, swales, conduits, or channels where localized 5
scouring is anticipated. 6
Implementation 7
The hydraulic design must be in accordance with Chapter 5 of this TSDM 8
Install riprap, grouted riprap, slope collars, headwalls, gabion mats, or energy 9
dissipater structures at the selected outlet 10
Designs are usually related to outlet flow rate and tailwater level 11
For the proper operation of aprons and rock pads, align the apron with the 12
receiving stream and keep it straight throughout its length 13
6.3.4 SLOPE PROTECTION SYSTEMS 14
Surface protection consists of permanent design measures that are used alone or in 15
combination to minimize erosion from sloped ground surfaces. Vegetated surface 16
protection is preferred as it offers advantages such as infiltration with lower runoff rates, 17
slower flow velocities, longer times of concentration, and lesser costs. However, where 18
site- or slope-specific conditions would prevent adequate establishment and 19
maintenance of a vegetative cover, hard surfacing should be considered. 20
Vegetated Surfaces 21
Description 22
A vegetated surface is a permanent perennial vegetative cover on all project 23
pervious areas. The purpose of a vegetated surface (from a water quality 24
perspective) is to prevent surface soil erosion, allow infiltration, and help to filter out 25
pollutants from surface flows. 26
Appropriate Applications 27
Vegetated surfaces should be established on areas of disturbed soil after 28
construction is completed and in areas where existing vegetation is not suitable as a 29
stabilization measure. Vegetated surfaces should only be considered for areas that 30
can support the selected vegetation long term. The vegetation type and application 31
should be selected in accordance with the guidance and recommendations of the 32
Guam Division of Aquatic and Wildlife Resources (DAWR). Another resource for 33
vegetation type and application guidelines is the USDA, National Resources 34
Conservation Service (NRCS). 35
6 - 16 August 2010
Implementation 1
The following steps are typically implemented by the designer: 2
Evaluate the site. The site evaluation should consider soil type, site 3
topography, and availability of equipment access. 4
Prepare grading plans as required to provide more efficient stabilization and 5
bed preparation for the plantings. Vegetated surfaces should be designed to 6
sheet flow conditions, and maximize contact time between water and 7
vegetated surfaces. This will enhance infiltration and pollutant removal 8
opportunities. 9
Select vegetation species to match site conditions and seasonal restraints in 10
accordance with NRCS‘s or DAWR‘s recommendations. Provide vegetation 11
application and seed bed preparation specifications and details. Require a 12
one-year maintenance guarantee from the contractor. Include temporary 13
irrigation, mulching and planting methods as may be necessary to match site 14
conditions and seasonal variations. 15
Site preparation may include removal and stockpiling of topsoil for later 16
replacement over the finished site grading. 17
Slope Roughening/Terracing/Rounding/Stepping 18
Roughening and terracing are techniques for creating furrows, terraces, serrations, stair-19
steps, or track marks on the soil surface to increase the effectiveness of temporary and 20
permanent soil stabilization practices. Slope rounding is a design technique to minimize 21
the formation of concentrated flows. 22
Use vegetated surfaces on embankment or cut slopes prior to the application of 23
temporary soil stabilization or permanent seeding. 24
Hard Surfaces 25
Description 26
Hard surfaces consist of placing concrete, rock, or rock and mortar slope protection. 27
The designer should consider the effects of increased runoff from impervious areas. 28
Appropriate Applications 29
Apply hard surfaces on soil areas where vegetation would not provide adequate 30
erosion protection. Hard surfaces are also considered where it is difficult to maintain 31
vegetation. 32
33
34
35
6 - 17 August 2010
Implementation 1
Rock Slope Protection: 2
Consists of riprap rock revetment courses along water courses. Angular rocks of 3
specified sizes are placed over filter fabric and/or gravel and used as riprap to 4
armor slopes, stream banks, etc. 5
Remove loose, sharp, or extraneous material from the slope to be treated. 6
Place underlayment fabric loosely over the surface so that the fabric conforms to 7
the surface without damage. Protect the fabric from puncture with a sand/gravel 8
overlay prior to placing the rock. Equipment or vehicles should not be driven 9
directly on the fabric. 10
Revetments along streams/rivers with potential for high velocities and/or stream 11
bed degradation should have an excavated toe trench with additional rock fill to 12
anchor the upper-rocked slope. 13
Alternatives to rock slope protection are rock-filled gabion baskets, gabion mats, 14
and vegetative stabilization. Vegetative-type slope protection measures are the 15
more preferred if conditions warrant. Vegetative measures can be combined with 16
rock and gabion slope protection measures. 17
Concreted Rock Slope Protection (Grouted Riprap): 18
Angular rocks of specified sizes are placed over engineering filter fabric 19
protected by a layer of sand/gravel mixture. 20
Lean concrete is placed into the rock interstices. Rock should be wetted 21
immediately prior to placement. Use of vibrators helps to work the concrete down 22
into the rock volume. 23
Grouted riprap is used where flow velocities exceed the loose riprap resistance, 24
or where larger riprap size and density suitable for the flow energy conditions is 25
not available. 26
Rock Blanket: 27
A rock blanket consists of round cobble rock placed as a landscape feature in 28
areas often inundated with water. The rock should be placed over a graded 29
sand/gravel filter layer and/or filter fabric. 30
Sacked Concrete Slope Protection: 31
Bags are filled with concrete mix and stacked against the slope to cure. Rebar 32
can be driven into the wet mix and bags. 33
Sacked concrete slope protection is aesthetically less desirable and is often 34
times used as an emergency measure to protect a higher-value structure. 35
6 - 18 August 2010
Slope Paving: 1
Slope paving is used almost exclusively below bridge decks at abutments. It 2
provides erosion control and soil stabilization in areas too dark for vegetation to 3
establish. It may be constructed of finish poured Portland Cement Concrete 4
(PCC), shotcrete, or masonry paving units. 5
Gabions: 6
Gabions are wire cages filled with rock. These units are then constructed into 7
structures of various configurations. They are an excellent alternative to riprap-8
type rock, where riprap rock of suitable density and size is not available. They 9
also allow for vegetative plantings using tree stakes and/or with a layer of soil for 10
seeding. They can be installed in locations of limited access by heavy machinery 11
using hand labor. 12
6.4 QUALITY TREATMENT BMPs 13
6.4.1 INFILTRATION BMPS 14
Infiltration BMPs for runoff treatment include the following: 15
IN.01 Bioinfiltration Pond IN.04 Infiltration Vault 16
IN.02 Infiltration Pond IN.05 Drywell 17
IN.03 Infiltration Trench 18
In addition to being the preferred method for flow control, infiltration is a preferred 19
method for runoff treatment, offering the highest level of pollutant removal. Treatment is 20
achieved through settling, biological action, and filtration. One important advantage to 21
using infiltration is that it recharges the groundwater, thereby helping to maintain dry 22
season base flows for streams. Infiltration also produces a natural reduction in stream 23
temperature, which is an important factor in maintaining a healthy habitat. Infiltration will 24
be the primary stormwater quality treatment BMP in the northern area of Guam, due to 25
the aquifer recharge requirements. 26
Infiltration facilities must be preceded by a pre-settling basin for removing most of the 27
sediment particles that would otherwise reduce the infiltrative capacity of the soil. 28
6.4.1.1 IN.01: BIOINFILTRATION POND 29
General Description 30
Bioinfiltration ponds, also known as bioinfiltration swales, combine filtration, soil 31
absorption, and uptake by vegetative root zones to remove stormwater pollutants by 32
percolation into the ground. 33
34
6 - 19 August 2010
Overflows are typically routed through an appropriate conveyance system to a higher 1
permeability infiltration BMP, such as a drywell or infiltration pond, or to a surface water 2
discharge point with flow control, as necessary. 3
Appropriate Applications 4
This BMP meets the runoff treatment objectives for all treated pollution-generating 5
impervious surfaces. Treatment is provided by a combination of the plant filtration and 6
root uptake of nutrients and the infiltration through soil. The infiltration capacity of these 7
facilities is usually insufficient to provide flow control to meet the criteria of Minimum 8
Requirement 6 as defined in Chapter 3 of this TSDM. Unless a very large area is 9
available for the shallow water depth required of a bioinfiltration pond, flow control must 10
be implemented using a different facility. Bioinfiltration ponds require moderately 11
permeable soil for proper function. In general, the site-suitability criteria are similar to 12
that of an infiltration pond. 13
Implementation 14
Pre-settling/Pre-Treatment 15
Pre-treatment for removal of suspended solids, trash, and oils is required to prevent 16
the bioinfiltration pond treatment soil from clogging. Pre-treatment BMPs can be 17
bioswales, wet bioswales, wet ponds, and sedimentation ponds. 18
Flows To Be Treated 19
Bioinfiltration ponds are designed as volume-based infiltration treatment facilities. 20
The runoff volume to be treated should be calculated using the guidelines in 21
Chapter 4 of this TSDM. 22
6 - 20 August 2010
1 Figure 6-4: Bioinfiltration Pond 2
6 - 21 August 2010
Geometry 1
Sizing methods for bioinfiltration ponds are the same as those for infiltration ponds 2
designed for runoff treatment, except for the following: 3
Drawdown time for the maximum ponded volume is 72 hours following the 4
design storm event. 5
The maximum ponded level is 6 in. prior to overflow to a drywell or other 6
infiltrative or overflow facility. 7
The swale bottom should be flat with a longitudinal slope less than 1 percent. 8
A concrete or riprap apron should be provided at the inlet to spread the flow 9
and keep vegetation from blocking the inlet. 10
The treatment zone depth of 6 in. or more should contain sufficient organics 11
and texture to ensure good vegetation growth. The average infiltration rate of 12
the 6-in. thick layer of treatment soil should not exceed 1 in. per hour for a 13
system relying on the root zone to enhance pollutant removal. Furthermore, a 14
maximum infiltration rate of 2.4 in. per hour is applicable. 15
Native grasses, adapted grasses, or other vegetation with significant root 16
mass should be used. 17
Pre-treatment may be used to prevent clogging of the treatment soil and 18
vegetation by debris, Total Suspended Solids (TSS), and oil and grease. 19
Site Design Elements 20
Initial excavation should be conducted to within 1 ft. of the final elevation of the floor 21
of the bioinfiltration pond. Defer final excavation to the finished grade until all 22
disturbed areas in the up-gradient drainage area have been stabilized or protected. 23
The final phase of excavation should remove all accumulated sediment. After 24
construction is completed, prevent sediment from entering the bioinfiltration pond by 25
first conveying the runoff water through an appropriate pre-treatment system, such 26
as a pre-settling basin. 27
Bioinfiltration ponds, as with all types of infiltration facilities, should generally not be 28
used as temporary sediment traps during construction. If a bioinfiltration pond is to 29
be used as a sediment trap, do not excavate to final grade until after the up-gradient 30
drainage area has been stabilized. Remove any accumulation of silt in the swale 31
before putting the swale into service. 32
Relatively light-tracked equipment is recommended for excavation to avoid 33
compacting the floor of the bioinfiltration pond. Consider the use of draglines and 34
track hoes. The bioinfiltration pond area should be flagged or marked to keep 35
equipment away. 36
6 - 22 August 2010
Setback Requirements 1
Setback requirements for bioinfiltration ponds are the same as those for infiltration 2
ponds. 3
Right-of-Way 4
Right-of-way requirements for bioinfiltration ponds are the same as those for 5
detention ponds. 6
Landscaping (Planting Considerations) 7
Use native or adapted grass species for the entire area of the bioinfiltration pond. 8
Maintenance Access Roads (Access Requirements) 9
Access requirements for bioinfiltration ponds are the same as those for infiltration 10
ponds. 11
6.4.1.2 IN.02: INFILTRATION POND 12
General Description 13
Infiltration ponds for stormwater quality treatment are earthen impoundments used for 14
the collection, temporary storage, and infiltration of incoming stormwater runoff to 15
groundwater. Infiltration ponds can also be designed as combination detention/infiltration 16
ponds for flow control where soil conditions limit full infiltration of the runoff. 17
Appropriate Applications: 18
Infiltration of runoff is the preferred method of stormwater quality treatment and flow 19
control. Runoff in excess of the infiltration capacity must be detained and released in 20
compliance with the flow control requirement described as part of Minimum 21
Requirement 6 in Chapter 3 of this TSDM. 22
Site-Suitability Criteria 23
Infiltration ponds require permeable soil conditions for proper function. For a site to be 24
considered suitable for an infiltration pond, the design infiltration rate must be at least 0.5 25
in. per hour. Infiltration can still be considered in the design if the infiltration rate is less, 26
but infiltration would be considered a secondary function in this case. 27
The infiltration rate to use for design should be established by site testing. Site infiltration 28
should be determined in-situ by a method that accurately estimates the infiltration rate of 29
the soil layer at the bottom of pond elevation. One such method is the ASTM D3385-30
Standard Test Method for Infiltration Rate of Soils in Field Using Double-Ring 31
Infiltrometer. At least three tests should be taken over each infiltration site. The design 32
infiltration rate will be the average value of the tests divided by a Safety Factor (SF). The 33
SF varies depending on the regular maintenance of the facility and the designer‘s 34
estimate of sedimentation loads that could plug and seal off the infiltration facility. For 35
6 - 23 August 2010
example, the SF will vary from a minimum of two for a facility with good maintenance 1
access serving a village street with curb and gutter inlets having relatively clean inflows, 2
to a SF of four for difficult to access sites with off-site inflows subject to higher 3
sedimentation and organic debris. 4
The base of all infiltration ponds should typically be 5 ft. above the seasonal high-water 5
mark, bedrock (or hardpan), or other low-permeability layer. This vertical distance may 6
be reduced to a minimum of 2 ft. if the following apply: 7
The groundwater mounding analysis, volumetric receptor capacity, and design of 8
the overflow or bypass structures are judged by the designer to be adequate to 9
prevent overtopping. 10
For the northern Guam aquifer recharge area, the bottom of the infiltration pond 11
must be at least 2 ft. above the top of the limestone bedrock (permeable aquifer 12
layer) to provide suitable soil filtration treatment prior to inflow to the aquifer. If 13
the infiltration pond bottom is less than 2 ft. above the limestone bedrock, then 14
another stormwater water quality treatment BMP must be incorporated prior to 15
flow to the infiltration pond. 16
The facility meets all other criteria listed in this BMP description. 17
Pre-settling/Pre-treatment 18
Infiltration ponds should follow a runoff treatment or pre-treatment facility to prevent 19
sediment build-up and clogging of the infiltrative soils. A pre-settling sedimentation 20
cell can be included in the infiltration pond design, as shown in Figure 6-5. If an 21
infiltration pond cannot meet the site-suitability criteria for treatment, a quality 22
treatment BMP must be provided prior to infiltration. 23
Design Flow Elements 24
Flows To Be Infiltrated 25
Site runoff should be infiltrated to the extent required in Minimum Requirement 5, 26
as described in Chapter 3 of this TSDM. Runoff in excess of the infiltration 27
capacity must be detained and released in compliance with the flow control 28
requirement described as part of Minimum Requirement 6. 29
For a site to be considered suitable for an infiltration pond, the design infiltration 30
rate must be at least 0.5 in. per hour. Infiltration can still be considered in flow 31
control facility design if the infiltration rate is less than this, but infiltration must be 32
considered a secondary function in that case. A pond should be designed to a 33
desirable depth of 3 ft. and a maximum depth of 6 ft., with a minimum freeboard 34
of 1 ft. above the design water level. 35
An infiltration pond must be designed using a single-event hydrograph model to 36
infiltrate the stormwater quality treatment volume out of the pond within 72 hours. 37
6 - 24 August 2010
An infiltration pond must be designed using a single-event hydrograph model to 1
detain and infiltrate the design frequency storm with an overflow for the higher 2
events. For locations without suitable overflow outlets, the 100-year frequency 3
storm should be detained and fully-infiltrated. 4
Outlet Control Structure 5
Runoff in excess of the infiltration capacity must be detained and released in 6
compliance with the flow control requirement described as part of Minimum 7
Requirement 6 as defined in Chapter 3 of this TSDM. 8
Refer to BMP FC.01 – Detention Pond for design of the outlet control structure. 9
6 - 25 August 2010
1 Figure 6-5: Infiltration Pond 2
6 - 26 August 2010
Site Design Elements 1
Setback Requirements 2
For infiltration facilities, the designer should evaluate the geotechnical report. The 3
report should address the adequacy of pond locations and recommend the 4
necessary setbacks from any steep slopes and building foundations. 5
Infiltration facilities should be set back at least 1000 ft. from drinking water wells, 6
and at least 100 ft. from septic tank drain field systems. 7
Infiltration facilities must be located at least 20 ft. downslope and 100 ft. upslope 8
from building foundations, and must be 5 ft. from any property line. 9
Flow Splitters 10
When designed to serve only as a runoff treatment facility, the pond may be 11
located offline. The facility must be designed to infiltrate all water quality volume 12
directed to it. All bypassed flow must be conveyed to a flow control facility, unless 13
it is directly discharged to an exempt water body. Note: Infiltration ponds 14
designed for flow control must be located on-line. 15
Emergency Overflow Spillway 16
An emergency overflow-type, non-erodible outlet or spillway must be constructed 17
to discharge flow in excess of the design flows to the downstream conveyance 18
system, as described in BMP FC.01-Detention Pond. Ponding depth, drawdown 19
time, and storage volume are calculated from the overflow elevation. 20
Structural Design Considerations 21
Geometry 22
The slope of the floor of an infiltration pond must not exceed 3 percent in any 23
direction. 24
Embankments 25
Requirements for infiltration pond embankments are the same as those for 26
BMP FC.01-Detention Pond. In addition to the testing for establishing the 27
design soil infiltration rate, the geotechnical investigation must include the 28
following: 29
Stability analysis of side slopes for ponds 30
Seepage analysis of any berms or dams 31
Liners 32
The floor of infiltration ponds can be covered with a 6-in. to 12-in. layer of 33
filter material such as coarse sand. Over time, incoming organic materials 34
6 - 27 August 2010
and fine soil particles will seal the surface of the pond, lowering the infiltration 1
rate. This filter layer can be readily replaced or cleaned as it becomes 2
clogged. 3
Site Design Elements 4
Initial excavation should be conducted to within 1 ft. of the final elevation of 5
the infiltration pond floor. Final excavation to the finished grade should be 6
deferred until all disturbed areas in the up-gradient drainage area have been 7
stabilized or protected. The final phase of excavation should remove all 8
accumulated sediment. 9
Infiltration ponds, as with all types of infiltration facilities, should generally not 10
be used as temporary sediment traps during construction. If an infiltration 11
pond is to be used as a sediment trap, it must not be excavated to final grade 12
until after the upstream drainage area has been stabilized. Any accumulation 13
of silt in the pond must be removed before the pond is put into service. 14
Low-ground-pressure equipment is recommended for excavation to avoid 15
compacting the floor of the infiltration pond. The use of draglines and track 16
hoes should be considered. The infiltration area should be flagged or marked 17
to keep equipment away. 18
Landscaping (Planting Considerations) 19
The interior of the infiltration pond, as well as surrounding berms, spoil areas, 20
borrow areas, and other disturbed areas, should be stabilized and planted, 21
preferably with suitable type grass. Without healthy vegetation, the surface 22
soil pores quickly plug. Vegetative planting mixes should use seed mix 23
suitable for the site and seasonal conditions. The actual seed mix, fertilizer, 24
and planting methods should be performed in accordance with NRCS or 25
DAWR recommendations. 26
Fencing 27
Fencing requirements for an infiltration pond are identical to those required 28
for BMP FC.01-Detention Pond. 29
Signage 30
Signage requirements for an infiltration pond are identical to those required 31
for BMP FC.01-Detention Pond 32
Maintenance Access Roads (Access Requirements) 33
Vehicle access must be provided for removal of sediment from the pre-34
treatment sedimentation cell (or forebay), and for maintenance of the outfall 35
6 - 28 August 2010
structure. Access roads and ramps should meet the minimum requirements 1
listed for BMP FC.01-Detention Pond. 2
Operation and Maintenance 3
Infiltration ponds, as with all BMPs, must have routine inspection and 4
maintenance designed into the life performance of the facility. They should be 5
inspected regularly during the wet season to ensure that the inlet and 6
overflow devices are not plugged and working correctly. They should also be 7
checked immediately after any large storm event and any damage should be 8
repaired immediately. The sedimentation cell should be checked annually for 9
sediment buildup and the accumulated sediment should be removed if it 10
exceeds 18 in. of depth. As part of the sediment removal process, the 11
disturbed soil areas and any other eroded spots should be re-graded to the 12
original grade and re-seeded. Infiltration facilities should also be checked at 13
least on an annual basis during the wet season for infiltration capacity. This 14
can be accomplished by observing that the time to empty after a storm event 15
is within the basic design requirements. 16
6.4.1.3 IN.03: INFILTRATION TRENCH 17
General Description 18
Infiltration trenches are long, narrow, stone-filled trenches used for the collection, 19
temporary storage, and infiltration of stormwater runoff to groundwater. They can be a 20
useful alternative for sites with constraints that make siting an infiltration pond difficult. 21
Infiltration trenches may be placed beneath parking areas, along the site periphery, or in 22
other suitable linear areas. For infiltration trench concept details, see Figures 6-6 23
through 6-11. 24
Appropriate Applications 25
The infiltration of runoff is the preferred method of runoff quality treatment and flow 26
control. Runoff in excess of the infiltration capacity must be otherwise detained and 27
released in compliance with the flow control requirement described as part of 28
Minimum Requirement 6 in Chapter 3 of this TSDM. 29
Site-Suitability Criteria 30
Site-suitability criteria are the same as those for BMP IN-01 Infiltration Ponds. 31
Pre-settling/Pre-treatment 32
Infiltration trenches should follow a runoff treatment or sedimentation (pre-treatment) 33
facility to limit sediment build-up and clogging of the trench. Where surface runoff is 34
routed directly to the infiltration trench, it should pass as sheet flow over a grassed 35
slope or through a biofiltration swale to help remove as much of the coarser 36
sediments as possible. 37
6 - 29 August 2010
Design Flow Elements 1
Flows To Be Infiltrated 2
The flows to be treated by an infiltration trench are identical to those for an infiltration 3
pond. 4
Overflow or Bypass 5
Because infiltration trenches are generally used for small drainage areas, an 6
emergency spillway is not necessary. However, a non-erosive overflow system 7
leading to a stabilized watercourse should be provided. If a suitable overflow site is 8
not available, then all the volume from the 100-year storm event should be detained 9
and infiltrated. 10
Outlet Control Structure 11
Outlet control structure requirements for an infiltration trench are identical to those for 12
an infiltration pond. 13
Flow Splitters 14
Flow splitter requirements for an infiltration trench are identical to those for an 15
infiltration pond. 16
Structural Design Considerations 17
Geometry 18
Infiltration trench-sizing methods use the volume of the voids of the rock, any 19
storage area above the rock, and additional volume within a pipe in the trench, as 20
appropriate, to detain the stormwater quality treatment and/or flow control volume 21
designed by routing the design storm inflow against the infiltration outflow. The 22
design method is essentially the same as that for BMP IN.02-Infiltration Pond, 23
where the design is based on in-situ infiltration testing. Note that in the Guam 24
limestone aquifer recharge area, the bottom of the infiltration volume must be at 25
least 2 ft. above the limestone bedrock surface. If less than 2 ft. of soil exists, 26
then the flows to the infiltration trench must be pretreated by another stormwater 27
quality treatment BMP. 28
Materials 29
Backfill Material 30
The backfill material for the infiltration trench should consist of clean aggregate 31
with a maximum diameter of 3 in. and a minimum diameter of 1.5 in. Void space 32
for the aggregate should be in the range of 30 percent to 40 percent. 33
6 - 30 August 2010
Geotextile Filter Fabric 1
An engineering geotextile material, which can be a sand/gravel mixture or natural 2
grassed soil condition, must encase all of the aggregate fill material under the top 3
6 in. to 12 in. of trench. Geotextile fabric with acceptable properties must be 4
carefully selected to avoid plugging. 5
Additional guidelines on geotextile filter fabric design and installation can be 6
found in FHWA publication NHI Course No. 13213, Geosynthetic Design and 7
Construction Guidelines, No. FHWA HI-95-038, April 1998. 8
Observation Well 9
An observation well should be installed at the lower end of the infiltration trench to 10
check water levels, draw-down time, and sediment accumulation, and to allow for 11
water-quality monitoring. The well should consist of a perforated PVC pipe 4 in. to 12
6 in. in diameter, with the top constructed flush with the ground elevation. For larger 13
trenches, a 12-in. diameter to 36-in. diameter well can be installed to facilitate 14
maintenance operations, such as pumping out trapped sediment. The top of the well 15
should be capped to discourage vandalism and tampering. See Figure 6-11 for 16
typical details. 17
Groundwater Issues 18
Groundwater issues for an infiltration trench are identical to those for an infiltration 19
pond. 20
Site Design Elements 21
Setback Requirements 22
Setback requirements for an infiltration trench are identical to those for an infiltration 23
pond. 24
Landscaping (Planting Considerations) 25
All disturbed earth surfaces and slopes should be vegetated with an erosion 26
protection type grass seed mix. The seed mix should be suitable for the site and 27
seasonal conditions. The actual seed mix, fertilizer, and planting methods should be 28
specified in accordance with NRCS or DAWR recommendations. 29
Maintenance Access Roads (Access Requirements) 30
Similar to all other hydraulic systems, the inlet/outlet structures that are more 31
susceptible to sedimentation and clogging by debris should be accessible for 32
maintenance by suction/rodding-type maintenance trucks. The observation well 33
should include a lockable cap or cover and be located for easy access by 34
maintenance personnel. 35
6 - 31 August 2010
1 Figure 6-6: Parking Lot Perimeter Trench Design 2
6 - 32 August 2010
1 Figure 6-7: Infiltration Trench System 2
6 - 33 August 2010
1 Figure 6-8: Median Strip Trench Design 2
3 Figure 6-9: Oversize Pipe Trench Design 4
6 - 34 August 2010
1 Figure 6-10: Underground Trench and Grit Chamber 2
3
4 Figure 6-11: Observation Wells 5
6 - 35 August 2010
Implementation 1
Trench Preparation 2
Excavated materials must be placed away from the trench sides to enhance trench 3
wall stability. Care should be taken to keep this material away from slopes, 4
neighboring property, sidewalks, and streets. It is recommended that this material be 5
covered with plastic. 6
Stone Aggregate Placement and Compaction 7
The stone aggregate should be placed in lifts and compacted using plate 8
compactors. As a rule of thumb, a maximum loose-lift thickness of 12 in. is 9
recommended. The compaction process ensures geotextile conformity to the 10
excavation sides, thereby reducing potential piping and geotextile clogging as well as 11
settlement problems. 12
Separation of Aggregate from Surrounding Soil 13
Natural or fill soils must not intermix with the stone aggregate. If the stone aggregate 14
becomes mixed with the soil, the stone aggregate must be removed and replaced 15
with uncontaminated stone aggregate. 16
Overlapping and Covering 17
The stone aggregate should be wrapped in a geotextile filter fabric. The fabric is 18
draped in the trench prior to the stone placement. Following stone aggregate 19
placement and compaction, the geotextile is then folded over the stone aggregate to 20
form a 12-in. minimum longitudinal overlap. When overlaps are required between 21
rolls, the upstream roll should overlap a minimum of 2 ft. over the downstream roll to 22
provide a shingled effect. 23
Voids Behind Geotextile 24
Voids between the geotextile and excavation sides must be avoided. The space left 25
by boulders or other obstacles removed from the trench walls is one source of such 26
voids. Natural soils should be placed in these voids at the most convenient time 27
during construction to ensure geotextile conformity to the excavation sides. Soil 28
piping, geotextile clogging, and possible surface subsidence can be avoided by this 29
remedial process. 30
Unstable Excavation Sites 31
Vertically excavated walls may be difficult to maintain in areas where the soil 32
moisture is high or where soft or cohesionless soils predominate. Trapezoidal rather 33
than rectangular cross sections may be needed. 34
Initial excavation should be conducted to within 1 ft. of the final elevation of the 35
infiltration pond floor. Final excavation to the finished grade should be deferred until 36
6 - 36 August 2010
all disturbed areas in the up-gradient drainage area have been stabilized or 1
protected. The final phase of excavation should remove all accumulated sediment. 2
Infiltration trenches, as with all types of infiltration facilities, should generally not be 3
used as temporary sediment traps during construction. If an infiltration trench is to be 4
used as a sediment trap, it must not be excavated to final grade until after the up-5
gradient drainage area has been stabilized. Any accumulation of silt in the trench 6
must be removed before the trench is put into service. 7
Operation and Maintenance 8
Infiltration trenches, as with all BMPs, must have routine inspection and maintenance 9
designed into the life performance of the facility. 10
6.4.1.4 IN.04: INFILTRATION VAULT 11
General Description 12
Infiltration vaults are typically large diameter, perforated pipe (tanks); pre-cast or cast-in-13
place reinforced concrete box structures; or bottomless reinforced concrete structures 14
used for temporary storage and the infiltration of stormwater runoff to groundwater. 15
These types of underground infiltration facilities can be a useful alternative for sites with 16
constraints that make locating an infiltration pond difficult. 17
Appropriate Applications 18
The infiltration of runoff is the preferred method of stormwater quality treatment and flow 19
control. Runoff in excess of the infiltration capacity must be detained and released in 20
compliance with the flow control requirement described as part of Minimum 21
Requirement 6 as defined in Chapter 3 of this TSDM. 22
Site-Suitability Criteria 23
The site-suitability criteria for an infiltration vault are the same as for infiltration 24
ponds. Infiltration vaults should not be constructed on slopes greater than 25 percent 25
(4H:1V). On slopes more than 15 percent, near tops of steep slopes or in landslide 26
hazard areas, a geotechnical investigation should be performed. 27
Pre-settling/Pre-Treatment 28
Infiltration vaults should follow a runoff treatment or sedimentation facility to prevent 29
sediment accumulation and clogging of the basin. 30
6 - 37 August 2010
1 Figure 6-12: BMP IN.04 - Infiltration Vault 2
Design Flow Elements 3
Flows To Be Infiltrated 4
The flows to be disposed to groundwater by infiltration vaults are the same as those 5
for BMP IN.02-Infiltration Ponds. 6
7
8
6 - 38 August 2010
Overflow or Bypass 1
For on-line vaults, a primary overflow must be provided to bypass flows larger than 2
the design storm event to the infiltration vault. See BMP FC.01-Detention Pond for 3
overflow structure types. For off-line facilities, a flow splitter may be used to direct the 4
stormwater quality treatment volume to the vault, while bypassing larger frequency 5
storms. 6
Outlet Control Structure 7
Outlet control structure requirements for infiltration vaults are the same as those for 8
BMP IN.02-Infiltration Pond. 9
Flow Splitters 10
Flow splitter requirements for infiltration vaults are the same as those for BMP IN.02-11
Infiltration Pond 12
Structural Design Considerations 13
Geometry 14
Infiltration vault geometric design requirements are similar to those for BMP 15
IN.02-Infiltration Pond. Where used only for stormwater quality treatment, the 16
storage volume sizing is based on a routing of the treatment volume inflow rate 17
with regard to the design infiltration outflow rate. For combination quality 18
treatment and flow control, the storage volume is based on the detention 19
requirements between the pre-development and post-development land cover 20
conditions for the design frequency storm. The infiltration rate is included as part 21
of the discharge flow rate for the detention sizing calculations. 22
Materials 23
All vaults (both perforated pipe and reinforced concrete) must meet structural 24
requirements for overburden support. Vaults located under roadways must meet 25
the live load requirements of the FHWA Standard Specifications for Construction 26
of Roads and Bridges on Federal Highway Projects (Standard Specification FP-27
03). Structural designs should be prepared by a Licensed Structural Engineer. 28
Bottomless vaults must be provided with footings placed on stable, well-29
consolidated native material and sized in consideration of overburden support, 30
traffic loading (assume maintenance traffic if vault is placed outside of right-of-31
way), and lateral soil pressures when the vault is dry. Infiltration vaults should not 32
be constructed in fill slopes unless a geotechnical analysis determines fill 33
stability. 34
Large diameter perforated pipe, bedding, and backfill should consist of washed, 35
drain rock extending at least 1 ft. below the bottom of the structure, at least 2 ft. 36
beyond the sides, and up to the top of the structure. The drain rock must be 37
6 - 39 August 2010
wrapped with geotextile filter fabric for separation from the native soil. If the drain 1
rock becomes mixed with soil, the effected rock material must be removed and 2
replaced with washed, drain rock to provide maximum infiltration effectiveness. 3
The perforations (holes) in the bottom half of the pipe should be 1 in. in diameter 4
and start at an elevation of 6 in. above the invert. The non-perforated portion of 5
the pipe in the lower 6 in. is intended for sediment storage to protect clogging of 6
the native soil beneath the structure. The number and spacing of the perforations 7
should be sufficient to allow complete infiltration of the soils with a safety factor of 8
2.0, without jeopardizing the structural integrity of the pipe. 9
The design must take into account the serviceability and design life of the 10
materials, as well as the structural stability, buoyancy, maintenance access, 11
access roads, and right-of-way. 12
Groundwater Issues 13
Groundwater issues for infiltration vaults are the same as those for BMP IN.02-14
Infiltration Pond. 15
Implementation 16
Initial excavation should be conducted to within 1 ft. of the final elevation of the 17
infiltration vault base. Final excavation to the finished grade should be deferred 18
until all disturbed areas in the up-gradient drainage area have been stabilized or 19
protected. The final phase of excavation should remove all accumulated 20
sediment. 21
Infiltration vaults, as with all types of infiltration facilities, should generally not be 22
used as temporary sediment traps during construction. If an infiltration vault is to 23
be used as a sediment trap, it must not be excavated to the final grade until after 24
the up-gradient drainage area has been stabilized. Any accumulation of silt in the 25
vault must be removed before the vault is put into service. 26
Relatively light-tracked equipment is recommended for excavation to avoid 27
compacting the soil beneath the base of the infiltration vault. The use of draglines 28
and track hoes should be considered. The infiltration area should be flagged or 29
marked to keep equipment away. 30
Operation and Maintenance 31
Infiltration vaults, as with all BMPs, must have routine inspection and maintenance 32
designed into the life performance of the facility. Access is required suitable for 33
suction/rodding-type maintenance trucks for removal of sediments, and for 34
maintenance personnel for routine inspections. 35
36
6 - 40 August 2010
6.4.1.5 IN.05: DRYWELL 1
General Description 2
Drywells are sub-surface concrete structures, typically pre-cast, that convey stormwater 3
runoff into the soil matrix. They can be used as stand-alone structures or as part of a 4
larger drainage system (for example, the overflow for a bioinfiltration pond). 5
Appropriate Applications 6
Drywells may be used for both stormwater quality treatment and flow control. Flow 7
splitters or overflow devices must be used for flows greater than the runoff treatment 8
design storm, and for flows greater than the design infiltration rate. 9
Drywells may be used for the same applications as BMP IN.02-Infiltration Ponds and 10
BMP IN.03-Infiltration Trench. 11
Pre-settling/Pre-Treatment 12
Drywells should follow a runoff treatment or sedimentation (pre-treatment) facility to 13
limit sediment build-up and clogging of the infiltration media. Where surface runoff is 14
routed directly to the drywell, it should pass as sheet flow over a grassed slope or 15
through a biofiltration swale to help remove as much of the coarser sediments as 16
possible. 17
Implementation 18
Inflow to infiltration facilities is calculated according to the methods described in 19
Chapter 4 of this TSDM. Storage volume should be provided within and above the 20
drywell as required for the necessary detention to contain the treatment volume and/or 21
flow control volume as required for infiltration of the design flows. The infiltration rate is 22
used in conjunction with the size of the storage area to design the facility. To prevent the 23
onset of anaerobic conditions, the infiltration facility must be designed to completely 24
drain 72 hours after the end of the design storm. 25
In general, an infiltration facility should have two discharge modes. The primary mode of 26
discharge is infiltration into the ground. However, when the infiltration capacity of the 27
facility is reached, a secondary overflow system is required to safely pass the flow from 28
storms larger than the design frequency. Overflows from an infiltration facility must 29
comply with Minimum Requirement 6 as defined in Chapter 3 of this TSDM. 30
Flows To Be Infiltrated 31
The flows to be disposed to groundwater by drywells are the same as those for BMP 32
IN.02-Infiltration Pond. 33
Overflow or Bypass 34
On-line systems should have an overflow provision to safely pass storm flows larger 35
than the design frequency. Drywells used off-line for stormwater quality treatment 36
6 - 41 August 2010
should use a flow splitter, so that flows larger than the treatment volume bypass the 1
inlet to the drywell. 2
3
Figure 6-13: BMP IN.05 - Drywell 4
Structural Design Considerations 5
Geometry 6
Typically, drywells are a minimum of 48 in. in diameter and are 7
approximately 5 ft. to 10 ft. or more deep 8
Filter fabric (geotextile) may need to be placed on top of the drain rock 9
and on trench or drywell sides before the drywell is backfilled to prevent 10
the migration of fines into the drain rock, depending on local soil 11
conditions 12
Drywells should be spaced no closer than 30 ft. center-to-center or twice 13
the structure depth in free-flowing soils, whichever is greater 14
Drywells should not be built on slopes greater than 25 percent (4H: 1V) 15
6 - 42 August 2010
Drywells should not be placed on or above a landslide hazard area or 1
slopes greater than 15 percent without a geotechnical evaluation 2
Groundwater Issues 3
Drywell bottoms should be a minimum of 5 ft. above the seasonal high- groundwater 4
level or impermeable soil layers. In northern Guam, this requirement is waived if 5
located above the limestone aquifer recharge area. However, if the discharge 6
infiltration level is less than 2 ft. above the limestone bedrock, the inflows must be 7
pre-treated through another stormwater quality treatment BMP. 8
Site Design Elements 9
Setback Requirements 10
The setback requirements for drywells are the same as those for BMP IN.02-11
Infiltration Ponds. 12
Signage 13
The requirements for signs are the same as those for BMP IN.02-Infiltration Ponds. 14
6.4.1.6 IN.05: PERMEABLE PAVEMENT 15
General Description 16
Permeable (porous or pervious) surfaces can be applied to non-pollution-generating 17
surfaces such as pedestrian/bike paths, raised traffic islands, and sidewalks. 18
Permeable surfaces with a media filtration sub-layer (such as sand or an amended 19
soil) could be applied to pollution generating surfaces (such as parking lots) for 20
calculating runoff treatment. Permeable surfaces allow stormwater to pass through 21
and infiltrate the soil below, thereby reducing the rate and volume of runoff 22
associated with conventional surfacing and fostering groundwater recharge. 23
The permeable concrete or asphalt pavement surface is an open-graded mix placed 24
in a manner that results in a high degree of interstitial spaces or voids within the 25
cemented aggregate. This technique demonstrates a high degree of absorption or 26
storage within the voids and infiltration to sub-soils. The pavement may be 27
permeable concrete, permeable asphalt, or manufactured systems such as 28
interlocking brick or a combination of sand and brick lattice. Geo-cell with geotextile 29
and aggregate material may also be considered for limited applications. 30
Appropriate Applications 31
Possible areas for use of these permeable surface materials include the following: 32
Sidewalks, bicycle trails, community trail/pedestrian path systems, or any 33
pedestrian-accessible paved areas (such as traffic islands) 34
6 - 43 August 2010
Vehicle access areas, including emergency stopping lanes, 1
maintenance/enforcement areas on divided highways, and facility 2
maintenance access roads 3
Public and municipal parking lots, including perimeter and overflow 4
parking areas 5
Permeable surface systems function as stormwater infiltration areas and temporary 6
stormwater retention areas that can accommodate pedestrians and light- to medium-7
load parking areas. They are applicable to both residential and commercial 8
applications, with the exception of heavy truck traffic. This combination of functions 9
offers the following benefits: 10
Captures and retains precipitation on-site 11
Mimics natural soils filtration throughout the pavement depth, underlying 12
sub-base reservoir, and native soils for improved groundwater quality 13
Eliminates surface runoff, depending on existing soil conditions 14
Greatly reduces or eliminates the need for an on-site stormwater 15
management system 16
Reduces drainage water runoff temperatures 17
Increases recharge of groundwater 18
Provides runoff treatment with a media filtration layer 19
Handling and placement practices for permeable surfaces are different from 20
conventional pavement placement. Unlike conventional pavement construction, it is 21
important that the underlying native or sub-grade soils be nominally consolidated to 22
prevent settling and minimize the effect of intentional or inadvertent heavy 23
compaction due to heavy equipment operation during construction. Consolidation 24
can be accomplished using static dual-wheel small mechanical rollers or plate 25
vibration machines. If heavy compaction does occur, then tilling may be necessary to 26
a depth of 2 ft. below the material placement. This would occur prior to subsequent 27
application of the separation and base layers. 28
Permeable surfaces are vulnerable to clogging from sediment in runoff. The following 29
techniques will reduce this potential: 30
Surface runoff. Permeable surfaces should not be located where turbid 31
runoff from adjacent areas can introduce sediments onto the permeable 32
surface. Designs should slope impervious runoff away from permeable 33
pavement installations to the maximum extent possible. 34
Diversion. French drains, or other diversion structures, may be designed 35
into the system to avoid unintended off-site runoff. Permeable systems 36
6 - 44 August 2010
can be separated using edge drain systems, turnpikes, and 0.15-foot-high 1
tapered bumps. 2
Slopes. Off-site drainage slopes immediately adjacent to the permeable 3
surface should be less than 5 percent to reduce the chance of soil loss 4
that would cause clogging. 5
Limitations 6
Suitable grades, subsoil drainage characteristics, and groundwater table conditions 7
require good multidisciplinary analysis and design. Proper construction techniques 8
and diligent field inspection during the placement of permeable surfaces are also 9
essential to a successful installation. 10
Installation works best with level, adjacent slopes — 1 percent to 2 11
percent — and on upland soils. Permeable surface installations are not 12
appropriate when adjacent draining slopes are 5 percent or greater. 13
An extended period of saturation of the base material underlying the 14
surface is undesirable. Therefore, the subsurface reservoir layer should 15
fully drain in a period of less than 36 hours. 16
The minimum depth from the bottom of the base course to bedrock and 17
seasonally high water table should be 3 feet, unless it is possible to 18
engineer a groundwater bypass into the system. 19
Examples of situations where the use of permeable surfaces is not currently 20
recommended include the following: 21
Roadway lanes because of a number of considerations (such as dynamic 22
loading, safety, clogging, and heavy loads). More study and experience 23
are needed before using permeable surfaces in these situations. Use of 24
any type of shoulder application whereby the retained moisture drains 25
away from the main line requires coordinated approval from materials, 26
roadway design, hydraulics, and maintenance support staff. 27
Areas where the permeable surface will be routinely exposed to heavy 28
sediment loading. 29
Areas such as maintenance yards that are subject, or potentially subject, 30
to higher pollutant loadings, spills, and piles of bulk materials (such as 31
sand or salt) should not incorporate permeable pavements. 32
Areas where the risk of groundwater contamination from organic 33
compounds is high (e.g., fueling stations, commercial truck parking areas, 34
and maintenance and storage yards). 35
6 - 45 August 2010
Within 100 feet of a drinking water well and within areas designated as 1
sole source aquifers. 2
Areas with a high water table or impermeable soil layer. 3
Within 100 feet up-gradient or 10 feet down-gradient from building 4
foundations. Closer up-gradient distances may be considered where the 5
minimum seasonal depth to groundwater lies below the foundation or 6
where it can be demonstrated that infiltrating water from the permeable 7
surface will not affect the foundation. 8
General Design Considerations 9
All projects considering the use of permeable surfaces should be further explored in 10
coordination with the Design, Materials Lab, Hydraulics, and Maintenance offices. 11
As long as runoff is not directed to the permeable asphalt from adjacent 12
surfaces, the estimated long-term infiltration rate may be as low as 0.1 13
inch/hour. Soils with lower infiltration rates should have underdrains to 14
prevent prolonged saturated soil conditions at or near the ground surface 15
within the pavement section. 16
For initial planning purposes, permeable surface systems will work well 17
on Hydrologic Soil Groups A and B and can be considered for Group C 18
soils. 19
Standard three-layer placement sections for Group D soils may not be 20
applicable. 21
For projects constructed on Group C and D soils, a minimum of three soil 22
gradation analyses or three infiltration tests should be conducted to 23
establish on-site soil permeability. Otherwise, a minimum of one such test 24
should be conducted for Group A and B soils to verify adequate 25
permeability. 26
Ideally, the base layer should be designed with sufficient depth to meet 27
flow control requirements (taking into account infiltration). If the infiltration 28
rate and base layer‘s recharge bed storage does not meet flow control 29
requirements, an underdrain system may be required. The underdrain 30
could be discharged to a bioretention area, dispersion system, or 31
stormwater detention facility. 32
Turbid runoff to the permeable surface from off-site areas is not allowed. 33
Designs may incorporate infiltration trenches or other options to ensure 34
long-term infiltration through the permeable surface. 35
6 - 46 August 2010
Any necessary boreholes must be installed to a depth of 10 feet below 1
the base of the reservoir layer, and the water table must be monitored at 2
least monthly for a year. 3
Infiltration systems perform best on upland soils. 4
On-site soils should be tested for porosity, permeability, organic content, and 5
potential for cation exchange. These properties should be reviewed when designing 6
the recharge bed of pervious surfaces. 7
Once a permeable surface site is identified, a geotechnical investigation should be 8
performed to determine the quantity and depth of borings/test pits required and any 9
groundwater monitoring needed to characterize the soil. 10
For on-site locations where subgrade materials are marginal, the use of a heavy-duty 11
geogrid placed directly on subgrade may be necessary. A sand layer is placed above 12
the heavy geogrid, followed by geotextile for drainage. 13
Design Flow Elements 14
For sizing purposes, use the following guidelines: 15
The bottom area of an ―infiltration basin‖ will typically be equivalent to the 16
area below the surrounding grade underlying the permeable surface. 17
Adjust the depth of this ―infiltration basin‖ so that it is sufficient to store the 18
required design volume. 19
Multiply this depth by a factor of five. This will determine the depth of the 20
gravel base underlying the permeable surface. This assumes a void ratio 21
of 0.20 — a conservative assumption. When a base material that has a 22
different porosity will be used, that value may be substituted to determine 23
the depth of the base. The minimum base depth is 6 inches, which allows 24
for adequate structural support of the permeable surface. 25
For a large, contiguous area of permeable surface, such as a parking lot, 26
the area may be designed with a level surface grade and a sloped 27
subgrade to prevent water buildup on the surface, except under extreme 28
conditions. Rare instances of shallow ponding in a parking lot are 29
normally acceptable. 30
For projects where ponding is unacceptable under any condition, the 31
surface of the parking lot may be graded at a 1 percent slope leading to a 32
shallow swale, which would function to ensure emergency drainage 33
(similar to an emergency overflow from a conventional infiltration pond). 34
However, the design depth of the base material must be maintained at all 35
locations. 36
37
6 - 47 August 2010
Maintenance Considerations 1
Permeable surfaces require more maintenance than conventional pavement 2
installations. The primary concern in maintaining the continued effectiveness of a 3
permeable surface system is to prevent the surface from clogging with fine 4
sediments and debris. 5
Materials 6
Permeable surfaces consist of a number of components — the surface pavement, an 7
underlying base layer, a separation layer, and the native soil or subgrade soil. An 8
overflow or underdrain system may need to be considered as part of the pavement‘s 9
overall design. 10
Surface Layer 11
The surface layer is the first component of a permeable system‘s design that creates 12
the ability for water to infiltrate through the surface. Permeable paving systems allow 13
infiltration of storm flows; however, the wearing course should not be allowed to 14
become saturated from excessive water volume stored in the aggregate base layer. 15
Portland Cement-Based Pervious Pavement Materials 16
The surface layer consists of specially formulated mixtures of Portland cement, 17
uniform open-graded coarse aggregate, and potable water. The depth of the surface 18
layer may increase from a minimum of 4 inches, depending on the required bearing 19
strength and pavement design requirements. The gradation required to obtain a 20
pervious concrete pavement is of the open-graded or coarse type (AASHTO Grading 21
No. 67, ¾ inch and lower). 22
Due to the relatively low water content of the concrete mix, an agent may be added 23
to retard concrete setup time. When properly handled and installed, pervious 24
pavement has a higher percentage of void space than conventional pavement 25
(approximately 12 percent to 21 percent), which allows rapid percolation of 26
stormwater through the pavement. The initial permeability can commonly exceed 200 27
inches per hour 28
Asphalt-Based Pervious Pavement Materials 29
The surface asphalt layer consists of an open-graded asphalt mixture. The depth of 30
the surface layer may increase from a minimum of 4 inches, depending on the 31
required bearing strength and pavement design requirements. 32
Pervious asphalt pavement consists of an open-graded coarse aggregate. The 33
pervious asphalt creates a surface layer with interconnected voids that provide a 34
high rate of permeability. 35
36
6 - 48 August 2010
Paving and Lattice Stones 1
Paving and lattice stones consist of a high-compressive-strength stone that may 2
increase from a minimum depth of 4 inches, depending on the required bearing 3
strength and pavement design requirements. When placed together, these paving 4
stones create a reinforced surface layer. An open-graded fine aggregate fills the 5
voids, which creates a system that provides infiltration into a permeable base layer. 6
This system can be used in parking lots, bike paths, or areas that receive common 7
local traffic. 8
Geo-Cell (PVC Containment Cell) 9
A Geo-Cell surface stabilization system consists of a high-strength, UV-resistant, 10
PVC-celled panel that is 4 inches thick. The celled panels can be filled with soil and 11
covered with turf by installing sod. Base gravel may also be used to fill the celled 12
panels. Both applications create a surface layer. 13
The Geo-Cell creates an interlock layer with interconnected voids that provide a high 14
rate of permeability of water to an infiltrative base layer. The common applications for 15
this system are on slopes and pedestrian/bike paths and in parking and low-traffic 16
areas. 17
Base Layer 18
The underlying base material is the second component of a permeable surface's 19
design. The base material is a crushed aggregate and provides the following: 20
A stable base for the pavement. 21
A high degree of permeability to disperse water downward through the 22
underlying layer to the separation layer. 23
A temporary reservoir that slows the migration of water prior to infiltration 24
into the underlying soil. 25
Base material is often composed of larger aggregate (1.5 to 2.5 inches) 26
with smaller stone (leveling or choker course) between the larger stone 27
and the wearing course. Typical void space in base layers ranges from 20 28
percent to 40 percent. 29
Depending on the target flow control standard and physical setting, 30
retention or detention requirements can be partially or entirely met in the 31
aggregate base. 32
Aggregate base depths of 18 to 36 inches are common depending on 33
storage needs, and they provide the additional benefit of increasing the 34
strength of the wearing course by isolating underlying soil movement and 35
imperfections that may be transmitted to the wearing course. 36
6 - 49 August 2010
Separation Layer 1
The third component of permeable systems is the separation layer. This layer 2
consists of a non-woven geotextile fabric and possibly a treatment media base 3
material. A geotextile fabric layer is placed between the base material and the native 4
soil to prevent migration of fine soil particles into the base material, followed by a 5
runoff treatment media layer if required. 6
For geotextile, see Standard Specifications. 7
For separation base material, see the FHWA manual ―Construction of 8
Pavement Subsurface Drainage Systems‖ (2002) for aggregate gradation 9
separation base guidance. 10
A treatment media layer is not required where subgrade soil is 11
determined to have a long-term infiltration rate less than 2.4 inches per 12
hour and a CEC of the subgrade soil that is at least 5 milliequivalents/100 13
grams of dry soil or greater. 14
If a treatment media layer is used, it must be distributed below the 15
geotextile layer and above the subgrade soil. The media can consist of a 16
sand filter layer or amended soil. Engineered amended soil layers should 17
be a minimum of 18 inches and incorporate compost, sphagnum peat 18
moss, or other organic material to provide a cation exchange capacity of 19
greater than or equal to 5 milliequivalents/100 grams of dry soil. 20
Gradations of the treatment media should follow base sizing. 21
Subgrade Soil 22
The underlying subgrade soil is the fourth component of pervious pavement. Runoff 23
infiltrates into the soil and moves to the local interflow or groundwater layer. 24
Compaction of the subgrade must be kept to an absolute minimum to ensure the soil 25
maintains a high rate of permeability while maintaining the structural integrity of the 26
pavement. 27
Liners 28
The primary purpose of a permeable pavement system is to promote infiltration. An 29
impervious liner will discontinue infiltration; therefore, a flow control credit is not 30
allowed and the surface is modeled as impervious. 31 32
6.4.2 DISPERSION: BMPS 33
Perhaps the single most promising and effective approach to mitigating the effects of 34
highway runoff in non-urbanized areas is to look for opportunities to use the existing 35
natural area capacity to remove pollutants. Natural dispersion requires that runoff cannot 36
become concentrated in any way as it flows into a preserved, naturally vegetated area. 37
6 - 50 August 2010
The preserved, naturally vegetated area must have topographic, soil, and vegetation 1
characteristics that provide for the removal of pollutants. Pollutant removal typically 2
occurs through a combined process of vegetative filtration and shallow surface 3
infiltration. Dispersion BMPs include: 4
RT.01 – Natural Dispersion 5
RT.02 – Engineered Dispersion 6
6.4.2.1 RT.01: NATURAL DISPERSION 7
General Description 8
Natural dispersion is the simplest method of flow control and stormwater quality runoff 9
treatment. This BMP can be used for impervious and pervious surfaces that are graded 10
so that runoff occurs by sheet flow. Natural dispersion uses the existing vegetation, soils, 11
and topography to effectively provide flow control and runoff treatment. It generally 12
requires little or no construction activity. Site selection is very important to the success of 13
this BMP. The pollutant-removal processes include infiltration into the existing soils and 14
through vegetation root zones, evaporation, and uptake and transpiration by the 15
vegetation. 16
The key to natural dispersion is that flows from the impervious area enter the natural 17
dispersion area as sheet flow. Because stormwater enters the dispersion area as sheet 18
flow, it only needs to traverse a narrow band of contiguous vegetation for effective 19
attenuation and treatment. The goal is to have the flows dispersed into the surrounding 20
landscape so that there is a low probability that any surface runoff will reach a flowing 21
body of water. 22
When modeling the hydrology of the project site and the threshold discharge area, the 23
designer should treat natural dispersion areas and their tributary drainage areas as 24
disconnected from the project site because they do not contribute flow to other flow 25
control or runoff treatment BMPs. 26
Applications and Limitations 27
Applications 28
Natural dispersion is ideal for highways and linear roadway projects 29
Natural dispersion helps maintain the temperature norms of stormwater 30
because it promotes infiltration, evaporation, and transpiration, and should 31
not have a surface discharge to a lake or a stream 32
Natural dispersion areas meet runoff treatment criteria set forth in Minimum 33
Requirement 5 as defined in Chapter 3 of this TSDM 34
Natural dispersion areas meet flow control criteria set forth in Minimum 35
Requirement 6 as defined in Chapter 3 of this TSDM 36
6 - 51 August 2010
Limitations 1
The effectiveness of natural dispersion relies on maintaining sheet flow to the 2
dispersion area, which maximizes soil and vegetation contact and prevents 3
short-circuiting due to channelized flow. If sheet flow cannot be maintained, 4
natural dispersion will not be effective. 5
Natural dispersion areas must be within the project right-of-way limits. Natural 6
dispersion areas must be protected from future development. DPW may 7
ultimately have to purchase right-of-way or easements to satisfy the criteria 8
for natural dispersion areas, but this should be the last option a designer 9
should choose. 10
Natural dispersion areas initially may cost as much as other constructed 11
BMPs (ponds or vaults) because right-of-way or easements often need to be 12
purchased, but long-term maintenance costs are lower. These natural areas 13
will also contribute to the preservation of native habitat and provide visual 14
buffering of the roadway. 15
Floodplains are not suitable areas for natural dispersion. 16
Site Design Elements 17
Siting Criteria 18
The key to natural dispersion is having vegetative land cover with an established root 19
zone, where the roots, organic matter, and soil macro-organisms provide macro-20
pores to reduce surface compaction and prevent soil pore sealing. The vegetative 21
cover also provides filtration and maintains sheet flow, reducing the chance for 22
erosion. The following public agency owned areas are considered appropriate 23
candidates for natural dispersion because they are likely to retain these vegetative 24
conditions over the long term: 25
Within DPWs right-of-ways 26
Village or Territorial-protected beautification areas 27
Parks 28
Other Government-owned lands 29
Where outside of the DPW right-of-way, the dispersion area will require 30
permanent easements to restrict any other use or development by the other 31
public agency. 32
33
34
35
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Design Flow Elements 1
Flows To Be Dispersed 2
The size of the natural dispersion area depends on the flow-contributing area and the 3
predicted rates of water loss through the dispersion system. The designer should 4
ensure the dispersion area is sufficient to dispose the runoff through infiltration, 5
evaporation, transpiration, and soil absorption. 6
Sheet Flow 7
The sheet flow dispersion criteria for natural dispersion areas are as follows: 8
The sheet flow path leading to the natural dispersion area should not be 9
longer than 150 ft. The sheet flow path is measured in the direction of the 10
flow and generally represents the width of the pavement area. 11
Pervious shoulders and side slopes are not counted in determining the sheet 12
flow path. 13
The longitudinal length of the dispersion area should be equivalent to the 14
longitudinal length of roadway that is contributing sheet flow. 15
The longitudinal pavement slope contributing flow to a dispersion area should 16
be less than 5 percent. The lateral pavement slope should be less than 17
8 percent. 18
Roadway side slopes leading to natural dispersion areas should be 19
25 percent (4H:1V) or flatter. Slopes steeper than 25 percent are allowed if 20
the existing side slopes are well-vegetated and show no signs of erosion 21
problems. 22
For any existing slope that will lead to a natural dispersion area, if evidence of 23
channelized flows (rills or gullies) is present, a flow spreading device should 24
be used before those flows are allowed to enter the dispersion area. 25
Sizing Criteria 26
Based on Soil Characteristics 27
The following criteria are specific to sheet flow dispersion on all NRCS Type A and 28
some Type B soils, depending on the saturated hydraulic conductivity rates: 29
For saturated hydraulic conductivity rates of 4 in. per hour or greater, and for 30
the first 20 ft. along the sheet flow path of impervious surface that drains to 31
the dispersion area, there must be 10 lateral ft. of dispersion area width. For 32
each additional foot of impervious surface along the sheet flow path that 33
drains to the dispersion area, 0.25 lateral ft. of dispersion area should be 34
provided. 35
6 - 53 August 2010
For dispersion areas that receive sheet flow from only disturbed pervious 1
areas (bare soil and non-native landscaping), for every 6 ft. along the sheet 2
flow path of disturbed pervious area, 1 lateral ft. width of dispersion area is 3
required. 4
Criteria for sheet flow dispersion on all NRCS Type C and Type D soils and some 5
Type B soils with saturated hydraulic conductivity rates of 4 in. per hour or less: 6
For every 1 ft. of contributing pavement width, a dispersion area width of 7
6.5 ft. is needed. 8
Figure 6-14 illustrates the configuration of a typical natural dispersion area relative to 9
the roadway 10
Structural Design Considerations 11
Geometry 12
Be well vegetated with established root zones 13
Have an average longitudinal slope of 6H:1V or flatter 14
Have an average lateral slope of 6H:1V or flatter for both the roadway side 15
slope and the natural area to be part of the natural dispersion area 16
Have infiltrative soil properties that are verified by geotechnical evaluation 17
Natural dispersion areas should have a separation of at least 2 ft. between the 18
existing ground elevation and the average annual maximum groundwater elevation. 19
This separation depth requirement applies to the entire limits of the dispersion area. 20
There should be no discernible continuous flow paths through the dispersion area. 21
Setback Requirements 22
Natural dispersion areas can extend beyond DPW‘s right-of-ways, provided that the 23
documentation on right-of-way plans ensures (through easements) that the 24
dispersion area is not developed in the future. Off-site dispersion areas will normally 25
be limited to other public agency lands. 26
Natural dispersion areas should be set back at least 1,000 ft. from drinking water 27
wells and 100 ft. from septic tank drain field systems. 28
The project must not drain onto adjacent off-site properties. Otherwise, a drainage 29
easement will be required or additional right-of-way purchased. 30
Signage 31
The limits of the natural dispersion area should be physically marked in the field, 32
both during and after construction. 33
6 - 54 August 2010
Signage helps ensure the natural dispersion area is not cleared or disturbed after the 1
construction project. 2
Construction Considerations 3
For the installation of dispersal BMPs and conveyance systems near dispersion areas, 4
the area that needs to be cleared or grubbed should be minimized. Maintaining plant 5
root systems is important for dispersion areas. 6
The area around dispersion areas should not be compacted. 7
To the maximum extent practicable, low-ground-pressure vehicles and equipment 8
should be used during construction. 9
Maintenance Considerations 10
Maintenance pullout areas should be considered to promote successful maintenance 11
practices at dispersion areas. Pullout areas should be large enough to accommodate a 12
typical maintenance vehicle. 13
6 - 55 August 2010
1 Figure 6-14: BMP RT.01 - Natural Dispersion 2
3
6 - 56 August 2010
6.4.2.2 RT.02: ENGINEERED DISPERSION 1
General Description 2
Engineered dispersion is similar to natural dispersion. This BMP can be used for 3
stormwater quality treatment and flow control at end of conveyance ditch or pipe 4
locations. The stormwater conveyance discharges by sheet flow through level-flow 5
spreaders across engineered dispersion areas. Engineered dispersion uses the existing 6
vegetation or landscaped areas, existing soils or engineered compost-amended soils, 7
and topography to effectively provide flow control and runoff treatment. Site selection is 8
very important to the success of this BMP. The pollutant-removal processes include 9
infiltration to the existing or engineered soils and through vegetation root zones, 10
evaporation, and uptake and transpiration by the existing vegetation or landscaped 11
areas. 12
Applications and Limitations 13
Applications 14
Engineered dispersion is ideal for highways and linear roadway projects that 15
collect and convey stormwater to discrete discharge points along the project 16
Engineered dispersion maintains the temperature norms of stormwater 17
because it promotes infiltration, evaporation, and transpiration, and should 18
not have a surface discharge to a lake or stream 19
Engineered dispersion areas meet the basic and enhanced runoff treatment 20
criteria set forth in Minimum Requirement 5 as defined in Chapter 3 of this 21
TSDM 22
Engineered dispersion areas meet flow control criteria set forth in Minimum 23
Requirement 6 as defined in Chapter 3 of this TSDM 24
Limitations 25
The effectiveness of engineered dispersion relies on maintaining sheet flow to the 26
dispersion area, which maximizes soil and vegetated contact and prevents short-27
circuiting due to channelized flow. If sheet flow cannot be maintained, engineered 28
dispersion will not be effective. 29
The project must ensure that the engineered dispersion area is not developed in the 30
future. This requires that the engineered dispersion be constructed within the project 31
right-of-way. Alternately, the dispersion could be placed on other suitable public 32
agency property, although an easement or agreement will be needed. Otherwise, 33
additional right-of-way acquisition may be necessary. 34
6 - 57 August 2010
Engineered dispersion areas may cost as much as other BMPs (ponds or vaults) 1
because right-of-way and easements may need to be purchased and compost-2
amended soils may need to be added. 3
Floodplains are not suitable areas for engineered dispersion. 4
Design Flow Elements 5
Flows To Be Dispersed 6
The required size of the engineered dispersion area depends on the area 7
contributing flow and the predicted rates of water loss through the dispersion system. 8
The design flow is the stormwater quality treatment volume and/or the flow control 9
design storm frequency. 10
Structural Design Considerations 11
Geometry 12
The average longitudinal slope of the dispersion area should not exceed 13
6H:1V 14
The average lateral slope of the dispersion area should not exceed 6H:1V 15
There should be no discernible flow paths through the dispersion area 16
There should be no surface water discharge from the dispersion area to a 17
conveyance system 18
Materials 19
Compost-amended soils should be generously applied to the dispersion areas. The 20
final organic content of the soil in the dispersion areas should be 10 percent. 21
Site Design Elements 22
Siting Criteria 23
The following areas are appropriate engineered dispersion areas because they are 24
likely to remain in their existing condition over the long term: 25
DPW right-of-way 26
Protected beautification areas 27
Agricultural areas 28
Parks 29
Other Government-owned forestlands 30
Rural areas with zoned densities of less than one dwelling unit per 5 acres 31
6 - 58 August 2010
Engineered dispersion areas should have infiltrative soil properties that are verified 1
by a geotechnical evaluation, and should not be sited above slopes greater than 2
20 percent. 3
Engineered dispersion areas should have a separation of at least 2 ft. between the 4
existing ground elevation and the average annual maximum groundwater elevation. 5
Sizing Criteria 6
Figure 6-15 illustrates a typical engineered dispersion area relative to the adjacent 7
roadway. 8
Concentrated runoff from the roadway and adjacent upstream areas (e.g., in a ditch 9
or cut slope) must be incrementally discharged from the conveyance system, such 10
as a ditch, gutter, or storm sewer, through cross culverts or at the ends of cut 11
sections. These incremental discharges of newly-concentrated flows should not 12
exceed 0.5 cfs for a single discharge point from the conveyance system for the 100-13
year runoff event. Where flows exceed this limit, the design of the associated 14
discharge point and associated flow spreader should be carefully designed to avoid 15
any erosion or surface gullying for the 100-yr storm. 16
Discharge points with up to 0.5 cfs discharge for the peak 100-year flow may use 17
rock pads or level flow spreaders (spreader swales or trenches) to disperse flows. 18
Discharge points with 0.5 cfs and larger discharge for the 100-year peak flow must 19
use level flow spreaders or swales to disperse flows. 20
Level flow spreaders must be designed to accept surface flows (free discharge) from 21
a pipe, culvert, or ditch end. They must be aligned perpendicular to the flow path 22
(see Section 6.6.3 for level flow spreader design guidelines). Sheet flow over the 23
level spreader berm or control weir should not exceed 4 in. in depth. Level flow 24
spreaders must have a minimum spacing of 50 ft. 25
Flow paths from adjacent discharge points must not intersect within the required flow 26
path lengths, and dispersed flow from a discharge point must not be intercepted by 27
another discharge point. 28
Engineered dispersion areas should be selected so that there is no chance for any 29
surface flow (flow not otherwise infiltrated) entering a stream, wetland, or other 30
waterbody. 31
The width of the dispersion area should be at least three times the width required to 32
infiltrate the flow depth over the weir from the flow spreader trench or swale. The 33
infiltration width should be conservatively calculated using Mannings Equation for 34
sheet flow over a natural grass ground cover (see design guidelines for the bioswale 35
and vegetated filter strip BMPs) to obtain the flow velocity. Use the flow depth at the 36
overflow weir crest for the calculations. The infiltration width corresponds to the time 37
needed to infiltrate that depth of water times the sheet flow velocity. The required 38
6 - 59 August 2010
dispersion area width is at least three times the calculated infiltration width. 1
Infiltration rates used for the calculations can be either determined by infiltration site 2
testing by a qualified Geotechnical Engineer, or using the lower end of the ranges for 3
infiltration rates as determined by the NRCS soil surveys for the particular soil types, 4
by ground cover, slopes, and hydrologic soil group. 5
Pipe or Ditch Conveyance System 6
Flows collected in a pipe or ditch conveyance system require energy dissipation and 7
dispersal at the end of the conveyance system before entering the dispersion area. 8
Setback Requirements 9
Engineered dispersion areas can extend beyond DPW‘s right-of-way, provided that 10
documentation on the right-of-way plans ensures (through easement or agreement) 11
that the dispersion area is not developed in the future. 12
Engineered dispersion areas should be set back at least 1,000 ft. from drinking water 13
wells and 100 ft. from septic tank drain fields. 14
If the project significantly increases flows to off-site properties, a drainage easement 15
may be required or right-of-way purchased. 16
Signage 17
The limits of the engineered dispersion area should be marked as a stormwater 18
management facility and should also be physically marked in the field (during and 19
after construction). 20
Signage helps ensure that the engineered dispersion area is not cleared or disturbed 21
after the construction project. 22
Construction Considerations 23
For the installation of dispersal BMPs and conveyance systems near dispersion areas, 24
the area that needs to be cleared or grubbed should be minimized. Maintaining plant 25
root systems is important for dispersion areas. 26
The area around dispersion areas should not be compacted. 27
To the maximum extent practicable, low-ground-pressure vehicles and equipment 28
should be used during construction. 29
Maintenance Considerations 30
Allow for off-road parking for maintenance vehicles/personnel. Pullout areas should be 31
large enough to accommodate a typical maintenance vehicle. 32
6 - 60 August 2010
1 Figure 6-15: BMP RT.02 - Engineered Dispersion Area 2
3
6 - 61 August 2010
6.4.3 DESIGN CRITERIA: BIOFILTRATION BMPS 1
Biofiltration BMPs are discussed in this chapter and include the following: 2
RT.03 – Vegetated Filter Strip (basic, narrow area, and compost-amended) 3
RT.04 –Biofiltration Swale 4
RT.05 –Wet Biofiltration Swale 5
RT.06 – Continuous Inflow Biofiltration Swale 6
RT.07 – Media Filter Drain 7
6.4.3.1 RT.03: VEGETATED FILTER STRIP 8
Vegetated filter strips are land areas of planted vegetation and amended soils situated 9
between the pavement surface and a surface water collection system, pond, wetland, 10
stream, or river. The term buffer strip is sometimes used interchangeably with vegetated 11
filter strip, but in this TSDM, buffer strip refers to an area of natural indigenous 12
vegetation that can be enhanced or preserved as part of a riparian buffer or stormwater 13
dispersion system. 14
Runoff treatment to remove pollutants can be best accomplished before concentrating 15
the flow. A vegetated filter strip provides a very efficient and cost-effective runoff 16
treatment option. Vegetated filter strips function by slowing runoff velocities and filtering 17
out sediment and other pollutants, and by providing some infiltration into underlying 18
soils. 19
Vegetated filter strips consist of gradually sloping areas that run adjacent to the 20
roadway. As highway runoff sheets off the roadway surface, it flows through the grass 21
filter. The flow can then be intercepted by a ditch or other conveyance system and 22
routed to a flow control BMP or outfall. 23
One challenge associated with vegetated filter strips is that sheet flow can sometimes be 24
difficult to maintain. Consequently, vegetated filter strips can be short-circuited by 25
concentrated flows, which create eroded rills or flow channels across the strips. This 26
results in little or no treatment of stormwater runoff. 27
The design approach for vegetated filter strips involves site design techniques to 28
maintain prescribed maximum sheet flow distances as well as to ensure adequate 29
temporary storage so that the design storm runoff is treated. There is limited ponding or 30
storage associated with vegetated filter strips unless soil amendments and sub-surface 31
storage are incorporated into the design to reduce runoff volumes and peak discharges. 32
Vegetated filter strips can also be used as a pre-treatment BMP in conjunction with 33
infiltration BMPs. The sediment and particulate pollutant load that could reach the 34
primary BMP is reduced by the pre-treatment, which in turn reduces maintenance costs 35
and enhances the pollutant-removal capabilities of the primary BMP. 36
6 - 62 August 2010
There are three methods described in this section for designing vegetated filter strips: 1
basic vegetated filter strips, Compost-Amended Vegetated Filter Strips (CAVFS), and 2
narrow-area vegetated filter strips. The narrow-area vegetated filter strip is the simplest 3
method to design; however, its use is limited to impervious flow paths less than 30 ft. If 4
space is available to use the basic vegetated filter strip design or the CAVFS, either of 5
the two designs should be used in preference to the narrow-area vegetated filter strip. 6
For flow paths greater than 30 ft., designers should follow the design method for the 7
basic vegetated filter strip or the CAVFS. 8
The basic vegetated filter strip is a compacted roadside embankment that is 9
subsequently hydroseeded. The CAVFS is a variation of the basic vegetated filter strip 10
that adds soil amendments to the roadside embankment. The soil amendments improve 11
infiltration characteristics, increase surface roughness, and improve plant sustainability. 12
The CAVFS design incorporates compost into the native soils. The CAVFS bed should 13
have a final organic content of 10 percent. Once permanent vegetation is established, 14
the advantages of the CAVFS are higher surface roughness, greater retention and 15
infiltration capacity, improved removal of soluble cationic contaminants through sorption, 16
improved overall vegetative health, and a reduction of invasive weeds. 17
Compost-amended systems have somewhat higher construction costs due to more 18
expensive materials, but require less land area for runoff treatment, which can reduce 19
overall costs. 20
Applications and Limitations 21
Vegetated filter strips (narrow area, CAVFS, and basic) can be used to meet runoff 22
quality treatment objectives or as part of a treatment train to provide pre-treatment for 23
removal of sediments. 24
Applications 25
Vegetated filter strips can be effective in reducing sediments and the pollutants 26
associated with sediments such as phosphorus, pesticides, or insoluble metallic 27
salts. 28
Because they do not pond water on the surface for long periods, vegetated filter 29
strips help maintain the temperature norms of the water and deter the creation of 30
habitat for disease vectors, such as mosquitoes. 31
In less urbanized areas, vegetated filter strips can generally be located on existing 32
roadside embankments, reducing the need for additional right-of-way acquisitions. 33
Designs can be modified to reduce runoff volumes and peak flows when needed or 34
desired to reduce right-of-way acquisitions. 35
36
6 - 63 August 2010
Limitations 1
If sheet flow cannot be maintained, vegetated filter strips will not be effective 2
Vegetated filter strips are generally not suitable for steep slopes or large 3
impervious areas that can generate high-velocity runoff 4
The use of vegetated filter strips can be impracticable in watersheds where 5
open land is scarce or expensive 6
For most project applications where less than 10 ft. of roadside embankment 7
is available for water quality treatment, the Media Filter Drain (MFD) (see 8
BMP RT.07), is a more suitable BMP option 9
Improper grading can render this BMP ineffective 10
Design Flow Elements 11
Flows To Be Treated 12
Vegetated filter strips must be designed to treat the runoff treatment flow rate 13
discussed as part of Minimum Requirement 5 as defined in Chapter 3 of this TSDM, 14
and the guidelines and criteria provided in this section. 15
Design Criteria and Specifications 16
Drainage Area Limitations 17
Vegetated filter strips are used to treat small drainage areas. Flow must enter the 18
vegetated filter strip as sheet flow, spread out over the length (long dimension 19
perpendicular to flow) of the strip, generally no deeper than 1 in. For basic vegetated 20
filter strips and CAVFS, the greatest flow path from the contributing area delivering 21
sheet flow to the vegetated filter strip should not exceed 150 ft. For the narrow-area 22
vegetated filter strip, the maximum contributing flow path should not exceed 30 ft. 23
The longitudinal slope of the contributing drainage area parallel to the edge of 24
pavement should be 2 percent or less. The lateral slope of the contributing drainage 25
area perpendicular to the pavement edge should be 5 percent or less. 26
Vegetated filter strips should be fully integrated within site designs, and should be 27
constructed outside of the natural stream buffer area, whenever possible, to maintain 28
a more natural buffer along the streambank. 29
Vegetated Filter Strip Geometry 30
Vegetated filter strips must provide a minimum residence time of nine minutes for full 31
water-quality treatment. 32
Vegetated filter strips can be used for pre-treatment to another water quality BMP. 33
Wherever a basic vegetated filter strip or CAVFS system cannot fit within the 34
available space, a narrow area vegetated filter strip system can be used solely as a 35
6 - 64 August 2010
pre-treatment device. A narrow area design should have a minimum width of 4 ft., 1
and should take advantage of all available space. 2
Basic vegetated filter strips should be designed for lateral slopes (along the direction 3
of flow) between 2 percent and 15 percent. Steeper slopes encourage the 4
development of concentrated flow; flatter slopes encourage standing water. 5
Vegetated filter strips should not be used on soils that cannot sustain a dense grass 6
cover with high flow retardance. In areas where lateral grades are between 15 7
percent and 25 percent, designers should consider using a CAVFS or an MFD (see 8
BMP RT.07). The MFD will usually require less treatment area to achieve the water 9
quality treatment objectives. 10
The minimum width of the vegetated filter strip generally is dictated by the design 11
method. 12
Both the top and toe of the slope should be as flat as possible to encourage sheet 13
flow and prevent erosion. 14
The Manning‘s n to be used in the vegetated filter strip design calculations depends 15
on the type of soil amendment and vegetation conditions used in construction of the 16
vegetated filter strip (see Table 6-3). 17
When the runoff treatment peak flow rate QWQ has been established, the design flow 18
velocity can be estimated using Manning‘s Equation to calculate the width of the 19
vegetated filter strip parallel to the direction of flow. 20
Water Depth and Velocity 21
The maximum depth of sheet flow through a vegetated filter strip for the runoff 22
treatment design flow rate is 1.0 in. The maximum flow velocity for the runoff 23
treatment design flow velocity is 0.5 ft. per second. 24
Maintain Sheet Flow Conditions 25
Sheet flow conditions from the pavement into the vegetated filter strip should be 26
maintained. A no-vegetation zone may help establish and maintain this condition. 27
In areas where it may be difficult to maintain sheet flow conditions, consider using 28
aggregate as a flow spreader. Aggregate should be placed between the pavement 29
surface and the vegetated filter strip. The aggregate should meet the specifications 30
for crushed base course listed in the Standard Specifications or other aggregate 31
should be used that provides the equivalent functionality. 32
If there are concerns that water percolated within the aggregate flow spreader may 33
exfiltrate into the highway prism, impervious geotextiles can be used to line the 34
bottom of the aggregate layer. 35
36
6 - 65 August 2010
Option Soil and Vegetation Conditions Manning’s n
1 Fully-compacted and hydroseeded 0.20
2 Compaction minimized and soils amended, hydroseeded 0.35
3
Compaction minimized; soils amended to a minimum 10% organic content, hydroseeded, grass maintained at 95% density and 4 in. in length through mowing, periodic re-seeding, and possible landscaping with herbaceous shrubs
0.40
4
Compost-amended vegetated filter strip: compaction minimized, soils amended to a minimum 10% organic content, and vegetated filter strip top dressed with ≥3 in. vegetated compost or compost/mulch (seeded or landscaped)
0.55
Table 6-3: Manning’s n for BMP RT.03 - Vegetated Filter Strips 1
Design Method 2
1. Determine the runoff treatment design flow (QWQ). 3
2. Calculate the design flow depth at Qvfs. The design flow depth is calculated 4
based on the length of the vegetated filter strip (same as the length of the 5
pavement edge contributing runoff to the vegetated filter strip) and the lateral 6
slope of the vegetated filter strip parallel to the direction of flow. Design flow 7
depth is calculated using a form of Manning‘s equation: 8
9
10 11
Equation 6-1 12
Where: 13
Qvfs = Vegetated filter strip design flow rate in cfs. 14
n = Manning‘s roughness coefficient. Manning‘s n can be adjusted by 15
specifying soil and vegetation conditions at the project site, as specified in 16
Table 6-3. 17
y = Design flow depth in ft., also assumed to be the hydraulic radius = 1.0 18
in., maximum = 0.083 ft. 19
L = Length of vegetated filter strip parallel to pavement edge in ft. 20
s = Slope of vegetated filter strip parallel to direction of flow in ft./ft. 21
Vegetated filter strip slopes should be greater than 2 percent and less 22
than 15 percent. Vegetated filter strip slopes should be made as shallow 23
as is feasible by site constraints. Gently sloping vegetated filter strips can 24
6 - 66 August 2010
produce the required residence time for runoff treatment using less space 1
than steeper vegetated filter strips. 2
Rearranging Equation 6-1 to solve for y yields: 3
4 5
Equation 6-2 6
If the calculated depth y is greater than 1 in., either adjust the vegetated filter 7
strip geometry or use other runoff treatment BMPs. 8
3. Calculate the design flow velocity passing through the vegetated filter strip at the 9
vegetated filter strip design flow rate. The design flow velocity (VWQ) is based on 10
the vegetated filter strip design flow rate, the length of the vegetated filter strip, 11
and the calculated design flow depth from Step 2: 12
13 14
Equation 6-3 15
Where: 16
VWQ = Design flow velocity in ft./s 17
y = Design flow depth in ft. 18
19
4. Calculate the vegetated filter strip width. The width of the vegetated filter strip is 20
determined by the residence time of the flow through the vegetated filter strip. A 21
nine-minute (540-second) residence time is used to calculate vegetated filter strip 22
width: 23
24 25
Where: 26
W = Vegetated filter strip width in ft. 27
T = Time in s 28
VWQ = Design flow velocity in ft./s 29
A minimum width of 8 ft. is recommended in order to ensure the long-term 30
effectiveness of the vegetated filter strip. 31
6 - 67 August 2010
Narrow Area Vegetated Filter Strip 1
As previously discussed, narrow-area vegetated filter strips are limited to impervious 2
flow paths less than 30 ft. For flow paths greater than 30 ft., designers should follow the 3
basic vegetated filter strip guidelines. The sizing of a narrow-area vegetated filter strip is 4
based on the width of the roadway surface parallel to the flow path of the vegetated filter 5
strip and the lateral slope of the vegetated filter strip. 6
1. Determine the width of the roadway surface parallel to the flow path 7
draining to the vegetated filter strip. Determine the width of the roadway 8
surface parallel to the flow path from the upstream to the downstream edge of 9
the impervious area draining to the vegetated filter strip. This is the same as the 10
width of the paved area. 11
2. Determine the average lateral slope of the vegetated filter strip. Calculate 12
the lateral slope of the vegetated filter strip (parallel to the flow path), averaged 13
over the total length of the vegetated filter strip. If the slope is less than 14
2 percent, use 2 percent for sizing purposes. The maximum lateral slope allowed 15
is 15 percent. 16
3. Determine the required width of the vegetated filter strip. Use Figure 6-16 to 17
size the vegetated filter strip. Locate the width of the impervious surface parallel 18
with the flow path on one of the curves; interpolate between curves as 19
necessary. Next, move along the curve to the point where the design lateral 20
slope of the vegetated filter strip is directly below. Read the vegetated filter strip 21
width to the left on the y axis. The vegetated filter strip must be designed to 22
provide this minimum width, W, along the entire stretch of pavement draining to 23
it. 24
Materials 25
Vegetation 26
Vegetated filter strips should be planted with grass that can withstand relatively high-27
velocity flows as well as wet and dry periods. Filter strips may also incorporate native 28
vegetation such as small herbaceous shrubs to make the system more effective in 29
treating runoff and providing root penetration into subsoils, thereby enhancing 30
infiltration. Use NRCS or DAWR recommendations for a mix of grasses and plants 31
suitable for the project site. 32
Site Design Elements 33
Maintenance Access Roads (Access Requirements) 34
Access should be provided at the upper edge of all vegetated filter strips to enable 35
maintenance of the gravel flow spreader and permit lawnmower entry to the 36
vegetated filter strip. 37
6 - 68 August 2010
1
2 Figure 6-16: Narrow-Area Vegetated Filter Strip Design 3
6.4.3.2 RT.04 : BIOFILTRATION SWALE 4
General Description 5
Biofiltration swales are vegetation-lined channels designed to remove suspended solids 6
from stormwater. The shallow, concentrated flow within these systems allows for the 7
filtration of stormwater by plant stems and leaves. Biological uptake, biotransformation, 8
absorption, and ion exchange are potential secondary pollutant-removal processes. The 9
design procedure is described below. 10
Design Flow Elements 11
Flows To Be Treated 12
Biofiltration swales should be designed for the stormwater quality treatment design 13
flow rate, QWQ. 14
Structural Design Considerations 15
Sizing Procedure 16
Preliminary Steps 17
1. Determine the stormwater quality treatment design flow rate (QWQ). See 18
Chapter 4 of this TSDM. 19
6 - 69 August 2010
2. Select a suitable site location having suitable grade and topographic 1
characteristics. 2
3. Establish the longitudinal slope of the proposed biofiltration swale (see 3
Table 6-5 for criteria). 4
4. Select a soil and vegetation cover suitable for the biofiltration swale. The 5
grass seed mix and planting requirements should be selected based on 6
NRCS or DAWR recommendations per site and seasonal conditions. 7
Design Steps 8
5. Select a trial depth of flow, Y (see Table 6-5). 9
6. Select a trial swale cross-sectional width. 10
7. Use Manning‘s Equation (Equation 6-5) to make an initial approximation 11
relating hydraulic radius and dimensions for the selected swale shape. 12
Use trial and error for various values of ‗Y‘ and ‗b‘ until Qbiofl = QWQ: 13
14 15
Equation 6-4 16
Where: 17
Qbiofil = Swale flow rate in cfs 18
A = Wetted area in ft.2 19
R = A/P = Hydraulic radius in ft. 20
P = Wetted perimeter in ft. 21
s = Longitudinal slope of swale in ft./ft. 22
n = Manning‘s coefficient (see Table 6-4) 23
Y = Swale flow depth in ft. 24
b = Swale bottom width in ft. 25
8. Compute the flow velocity at Qbiofil: 26
27 28
Equation 6-5 29
Where: 30
Vbiofil = Flow velocity at Qbiofil in ft./s 31
6 - 70 August 2010
If Vbiofil > 1.0 ft./s, increase width (b) or investigate ways to reduce Qwq, 1
and then repeat steps 3, 4, and 5 until Vbiofil ≤ 1.0 ft./s. A velocity greater 2
than 1.0 ft./s will flatten grasses, thus reducing filtration. 3
9. Compute the swale length, L in ft.: 4
5
L = Vbiofil t (60 s/min.) 6
Equation 6-6 7
Where: 8
t = Hydraulic residence time (a minimum of nine minutes. for basic 9
biofiltration swales) 10
10. If there is not sufficient space for the biofiltration swale, consider the 11
following solutions: 12
Divide the site drainage to flow to multiple biofiltration swales 13
Use infiltration or dispersion to provide lower QWQ 14
Alter the design depth of flow if possible (see Table 6-5) 15
Reduce the developed surface area to gain space for the 16
biofiltration swale 17
Reduce the longitudinal slope by meandering the biofiltration 18
swale 19
Nest the biofiltration swale within or around another stormwater 20
BMP 21
22
23
6 - 71 August 2010
1 Figure 6-17: BMP RT.04 - Biofiltration Swale Plan 2
6 - 72 August 2010
1 Figure 6-18: BMP RT.04 - Biofiltration Swale Section 2
3
Soil and Cover Manning's Coefficient
Grass-legume mix on compacted native soil 0.20
Grass-legume mix on lightly compacted, compost-amended soil
0.22
Grass-legume mix on lightly compacted, compost-amended soil with surface roughness features
0.35
Table 6-4: Manning’s Coefficient in Basic, Wet, and Continuous Bioinfiltration Swales 4
5
Design Parameters
Basic Biofiltration Swale Wet Biofiltration Swale
Continuous Inflow Bioinfiltration Swale
Longitudinal slope 0.010–0.060 1 ft./ft.
0.015 ft. or less per ft.
Same as basic swale
Maximum velocity 1 ft./s at Qbiofil Same as basic swale
Same as basic swale
6 - 73 August 2010
Design Parameters
Basic Biofiltration Swale Wet Biofiltration Swale
Continuous Inflow Bioinfiltration Swale
Maximum water depth at Qbiofil, ‗Y‘
2 in. if swale mowed frequently; 4 in. if mowed infrequently or inconsistently. For dry land grasses, set depth to 3 in.
4 in. Same as basic swale
Manning coefficient, ‗n‘, at Qbiofil
See Table 6-4 Same as basic swale
Same as basic swale
Bed width, ‗b‘ 2ft.2 - 10 ft.
2 2 ft. - 25 ft.
Same as basic swale
Freeboard height 1 ft. for the peak conveyance flow rate (Qconvey)
3
Same as basic swale
Same as basic swale
Minimum length, ‗L‘ 100 ft. Same as basic swale
Same as basic swale
Maximum side slope (for trapezoidal cross section)
3H:1V Same as basic swale
Same as basic swale
Table 6-5: Bioinfiltration Swale Sizing Criteria 1
1. For basic biofiltration swale on slopes less than 1.5 percent, install an underdrain system (see Figure 6-2 19) 3
Backfill should be covered by at least 4 in. of amended soil or topsoil. Install the low-flow drain 6 in. deep in 4 the soil (see Figure 6-19). Underdrains can be made of 6-in. Schedule 40 PVC perforated pipe with 6 in. of 5 drain gravel on the pipe. The gravel and pipe must be enclosed by geotextile fabric. The pipe outlet should 6 be protected from erosion. 7
2. Multiple parallel swales can be constructed when the calculated swale bottom width exceeds 10 ft. using a 8 divider berm or concrete curb. 9
3. Qconvey should be based on the peak capacity of the incoming conveyance system to the biofiltration 10 swale. For off-line bioswales, this will be the design QWQ. For on-line bioswales this should be the maximum 11 capacity of the conveyance system, but not less than Q25-yr. 12
13
6 - 74 August 2010
1 Figure 6-19: Bioinfiltration Swale Underdrain Detail 2
6 - 75 August 2010
1 Figure 6-20: Bioinfiltration Swale Details 2
3
6 - 76 August 2010
Freeboard Check (FC) 1
A Freeboard Check (FC) must be performed for the combination of highest expected 2
flow and least vegetation coverage and height. The FC is not necessary for biofiltration 3
swales that are located off-line from the primary conveyance and detention system (that 4
is, when flows in excess of QWQ bypass the biofiltration swale). Off-line is the preferred 5
configuration of biofiltration swales. Ensure swale depth exceeds flow depth at Qconvey by 6
a minimum of 1 ft. (1-ft. minimum freeboard). 7
Site Design Elements 8
Install level spreaders at the head of the biofiltration swale in swales that are 6 ft. or 9
greater in bottom width. Include sediment cleanouts at the head of the swale as needed. 10
It is recommended that swales with a bottom width in excess of 6 ft. or greater have a 11
level spreader for every 50 ft. of swale length. 12
Use energy dissipaters for swales on longitudinal slopes exceeding 2.5 percent. 13
Specify that topsoil extends to at least an 8-in. depth, unless an underdrain system is 14
needed (see Table 6-5). 15
To improve infiltration on longitudinal slopes less than 1.5 percent, ensure the swale bed 16
material contains a sand percentage greater than 70 percent (greater than 70 percent by 17
weight retained on the No. 40 sieve) before organic amendments are added. 18
Provide an access road for maintenance vehicles. The width of access should normally 19
be 20 ft., but in no case should it be less than 12 ft. The access road should be surfaced 20
with a permeable surface suitable for heavier vehicular loadings such as crushed gravel, 21
vegetative paver blocks, or permeable pavement. The access road should provide 22
access for cleaning the inlet structures and extend along one side of the swale. The 23
length of the access road should be at least one-half of the length of the swale to 24
accommodate removal of accumulated sediments in the upper area. 25
Landscaping (Planting Considerations) 26
Select grasses suitable for the site conditions in accordance with DAWR‘s 27
recommendations 28
Select fine, turf-forming grasses where moisture is appropriate for growth 29
Plant wet-tolerant species for wet site locations 30
Stabilize soil areas upslope of the biofiltration swale to prevent erosion and 31
excessive sediment deposition 32
Apply seed using a hydroseeder or broadcaster 33
34
35
6 - 77 August 2010
Construction Criteria 1
Do not put the biofiltration swale into operation until areas of exposed soil in 2
the contributing drainage catchment have been sufficiently stabilized 3
Keep effective erosion and sediment control measures in place until the 4
swale vegetation is established 5
Avoid over-compaction during construction 6
Grade biofiltration swales to attain uniform longitudinal and lateral slopes 7
6.4.3.3 RT.05: WET BIOFILTRATION SWALE 8
General Description 9
A wet biofiltration swale is a variation of a basic biofiltration swale for use where the 10
longitudinal slope is slight, water tables are high, or continuous base flow is likely to 11
result in saturated soil conditions. Where saturation exceeds about two continuous 12
weeks, typical grasses die; so vegetation specifically adapted to saturated soil conditions 13
is needed. This type of vegetation, in turn, requires modification of several of the design 14
parameters for the basic biofiltration swale to remove low concentrations of pollutants, 15
such as sediments, heavy metals, nutrients, and petroleum hydrocarbons. 16
Applications and Limitations 17
Wet biofiltration swales are applied where a basic biofiltration swale is desired but not 18
allowed or advisable because of one or more of the following conditions: 19
The swale is on thin soils with low permeability on flat slopes 20
The swale is downstream of a detention pond providing flow control 21
Saturated soil conditions are likely because of seeps, high groundwater, or base 22
flows on the site 23
Longitudinal slopes are slight (generally less than 1.5 percent), and ponding is 24
likely 25
Design Flow Elements 26
Flows To Be Treated 27
Wet biofiltration swales should be designed to treat the runoff treatment flow rate 28
discussed as part of Minimum Requirement 5 as defined in Chapter 3 of this TSDM. 29
Overflow or Bypass 30
To accommodate flows exceeding the stormwater quality treatment flow rate, the 31
following two design options are available: 32
6 - 78 August 2010
1. A high-flow bypass can be installed for flows greater than the runoff treatment 1
design flow to protect wetland vegetation from damage. Unlike grass, wetland 2
vegetation does not quickly regain an upright attitude after being flattened by 3
high flows. New growth, usually from the base of the plant and often taking 4
several weeks, is required for the grass to regain its upright form. The bypass 5
may be a flow-splitter structure for a storm drain system, or an overflow type 6
open channel parallel to the wet biofiltration swale. 7
2. Alternatively, swale bottom width may be tripled to accommodate high flows. 8
The following features must be included: 9
An energy dissipater and level spreader must be set at the head of the 10
swale to ensure the equal distribution of influent and reduce the potential 11
for scour. Gravel filter berms must be placed every 30 ft. across the full 12
width of the swale. The minimum required swale length should not include 13
the length occupied by gravel filter berms. 14
If the calculated width to convey high flows exceeds 25 ft., then a high 15
flow channel with a bed elevation 0.5 ft. above the swale depth can be 16
constructed. The high-flow channel can be either planted or rock-lined. 17
Structural Design Considerations 18
Geometry 19
Use the same design approach as for basic biofiltration swales, except add the 20
following: 21
Extended wet season flow adjustment. If the swale is downstream of a 22
detention pond providing flow control, multiply the treatment area (bottom width 23
times length) of the swale by two and readjust the swale length or width to 24
provide an equivalent area. Maintain a 5:1 length-to-width ratio. 25
Intent: The treatment area of swales following detention ponds needs to be 26
increased because of the differences in vegetation established in a constant flow 27
environment. Flows following detention are much more prolonged. These 28
prolonged flows result in more stream-like conditions than are typical for other 29
wet biofiltration situations. Because vegetation growing in streams is often less 30
dense, an increase in treatment area is needed to ensure equivalent pollutant 31
removal is achieved in extended flow situations. 32
Swale geometry. Use the same geometry specified for basic biofiltration swales 33
(see Table 6-5), except for the following modifications: 34
The bottom width may be increased to 25 ft. maximum, but a length-to 35
width ratio of 5:1 must be maintained. No longitudinal dividing berm is 36
needed. Note: The minimum swale length is 100 ft. 37
6 - 79 August 2010
If longitudinal slopes are greater than 1.5 percent, the wet swale must be 1
stepped so that the slope within the stepped sections averages 1.5 2
percent or less. Steps may be made of concrete or block walls, short 3
riprap sections, or similar structures. Steps must be designed to prevent 4
scour or undermining on the downstream side of the step. No underdrain 5
or low-flow drain is required. 6
Water depth and base flow. Use the same criteria specified for basic 7
biofiltration swales, except that the design water depth must be 4 in. for all 8
wetland vegetation selections, and no underdrains or low-flow drains are 9
required. 10
Flow velocity, energy dissipation, and flow spreading. Use the same criteria 11
specified for basic biofiltration swales. 12
Site Design Elements 13
Landscaping (Planting Considerations) 14
Use the same design considerations specified for basic biofiltration swales, except 15
for the following modifications: 16
In general, it is best to plant several species to increase the likelihood that at 17
least some of the selected species will find growing conditions favorable. 18
A wetland seed mix may be applied by hydroseeding, but if coverage is poor, 19
planting rootstock or nursery stock is required. Poor coverage is considered 20
to be more than 30 percent bare area through the upper two-thirds of the 21
swale after four weeks. 22
Use plant species suitable for a wetland environment in accordance with 23
NRCS or DAWR recommendations. 24
Construction Considerations 25
Use the same construction considerations specified for basic biofiltration swales. 26
27
6 - 80 August 2010
1 Figure 6-21: BMP RT.05 - Wet Biofiltration Swale 2
6.4.3.4 RT.06: CONTINUOUS INFLOW BIOFILTRATION SWALE 3
General Description 4
In situations where water enters a biofiltration swale continuously along the side slope 5
rather than discreetly at the head, a different design approach — the continuous inflow 6
biofiltration swale — is needed. The basic swale design is modified by increasing swale 7
length to achieve an equivalent average hydraulic residence time. 8
Applications and Limitations 9
A continuous inflow biofiltration swale is used when inflows are not concentrated, such 10
as locations along the shoulder of a road without curbs. This design may also be used 11
where frequent, small-point flows enter a swale, such as through curb inlet ports spaced 12
at intervals along a road or from a parking lot with frequent curb cuts. In general, no inlet 13
port should carry more than approximately 10 percent of the flow. 14
A continuous inflow swale is not appropriate where significant lateral flows enter a swale 15
at some point downstream from the head of the swale. In this situation, the swale width 16
and length must be recalculated from the point of confluence to the discharge point, in 17
order to provide adequate treatment for the increased flows. 18
6 - 81 August 2010
Design Considerations 1
Use the same considerations specified for BMP RT.04–Biofiltration Swale (see 2
Table 6-5), except for the following: 3
For the design flow, include runoff from the pervious side slopes draining to 4
the swale along the entire swale length. 5
If only a single design flow is used, use the flow rate at the outlet. The goal is 6
to achieve an average residence time through the swale of nine minutes. 7
Assuming an even distribution of inflow into the side of the swale, double the 8
hydraulic residence time to a minimum of 18 minutes. 9
For continuous inflow biofiltration swales, plant interior side slopes above the 10
runoff treatment design elevation in grass. Landscape plants or groundcovers 11
other than grass generally should not be used between the runoff inflow 12
elevation and the bottom of the swale. 13
Intent: The use of grass on interior side slopes reduces the chance of soil erosion 14
and transfer of pollutants from landscape areas to the biofiltration treatment area. 15
After the cross-sectional size of the biofiltration swale is determined using Qbiofil, 16
complete the following steps. This is to account for the hydraulic residence time of 17
flow moving through the vegetated side slopes (3H:1V or shallower and slope length 18
>5 ft.) at various points along the length of the swale. 19
1. Break the drainage basin of the swale into areas so that no area contributes 20
more than 20 percent of the flow. Include only those areas that discharge 21
sheet flow to the vegetated side slopes and biofiltration swale. 22
2. Determine the velocity of flows through each vegetated side slope, Vn,ss (feet 23
per second), for each of the contributing areas by completing steps 1 24
through 3 of the vegetated filter strip design methodology. 25
3. Determine the hydraulic residence time within each vegetated side slope, tss 26
(second), for each area using: 27
Ln,ss/Vn,ss = tn,ss 28
Where: 29
Ln,ss = Length of vegetated side slope of the nth swale sub-basin in ft. 30
4. Determine the weighted mean hydraulic residence time, tmean,ss, for all flows 31
passing through vegetated side slopes using: 32
[Q1(tss,1)+Q2(tss,2)+….+Qn(tss,n)]/Qtotal,ss= tmean,ss 33
34
35
6 - 82 August 2010
Where: 1
Qn = Flow rate for nth contributing area in cfs 2
Qtotal,ss = Total flow that passes through all of the vegetated side 3
slopes in cfs 4
5. Multiply tmean,ss by R 5
Where: 6
tmean,ss x R = tadj 7
R = Qtotal,ss / Qbiofil 8
Qbiofil = QWQ (for the properly-sized swale section) 9
6. If the head of the swale is located downstream of the last contributing 10
vegetated side slope section, subtract tadj from 540 seconds (= nine minutes) 11
to determine the tdesign. If the swale is located along the entire toe of the 12
contributing vegetated side slope, subtract tadj from 1,080 seconds (=18 13
minutes) to determine tdesign. Note: In the latter case, the swale must be at 14
least as long as the contributing vegetated side slopes. 15
7. Using the downstream flow rate (Qbiofil), determine the velocity through the 16
swale, and use tdesign, calculated in Step 6, to determine the total swale length 17
required. Make any necessary adjustments to ensure the criteria in Table 6-5 18
are met. 19
6 - 83 August 2010
1 Figure 6-22: BMP RT.06 - Continuous Inflow Bioinfiltration Swale 2
3
4
6 - 84 August 2010
6.4.3.5 RT.07: MEDIA FILTER DRAIN 1
General Description 2
The Media Filter Drain (MFD) is a linear flow-through stormwater runoff treatment device 3
that can be sited along highway side slopes (conventional design), borrow ditches, or 4
other linear depressions. The MFD can be used where available right-of-way is limited, 5
sheet flow from the highway surface is feasible, and lateral gradients are generally less 6
than 25 percent (4H:1V). The MFD is also suitable as a phosphorus removal treatment 7
BMP, where discharge is to an Environmental Protection Agency (EPA)-designated 8
phosphorus protected water body. 9
MFDs have four basic components: a gravel no-vegetation zone, a grass filter strip, the 10
MFD mix bed, and a conveyance system for flows leaving the MFD mix. This 11
conveyance system usually consists of a gravel-filled underdrain trench or an underlying 12
layer of crushed gravel for free-side slope discharge. This layer of gravel must be porous 13
enough to allow treated flows to freely drain away from the MFD mix. Typical MFD 14
configurations are shown in Figures 6-23 through 6-26. 15
Functional Description 16
The MFD removes suspended solids, phosphorus, and metals from highway runoff 17
through physical straining, ion exchange, carbonate precipitation, and biofiltration. 18
Stormwater runoff is conveyed to the MFD through sheet flow over a vegetation-free 19
gravel zone to ensure sheet dispersion and provide some heavier particle pollutant 20
trapping. Next, a grass filter strip (planted in topsoil amended with compost) is 21
incorporated into the top of the fill slope to provide pre-treatment, further enhancing 22
filtration and extending the life of the system. The runoff is then filtered through a bed of 23
porous, alkalinity-generating granular medium — the MFD mix. MFD mix is a fill material 24
composed of crushed rock (sized by screening), dolomite, gypsum, and perlite. The 25
dolomite and gypsum additives serve to buffer acidic pH conditions and exchange light 26
metals for heavy metals. Perlite is incorporated to improve moisture retention, which is 27
critical for the formation of biomass epilithic biofilm to assist in the removal of solids, 28
metals, and nutrients. Treated water drains from the MFD mix bed into the conveyance 29
system (underlying free draining crushed gravel, with under drain as applicable) below 30
the MFD mix. Geotextile filter fabric is placed between to separate the underside of the 31
MFD mix bed from the underlying gravel material. 32
The underdrain trench is an option for hydraulic conveyance of treated stormwater to a 33
desired location, such as a downstream flow control facility or stormwater outfall. 34
It is critical to note that water must sheet flow across the MFD. 35
36
6 - 85 August 2010
1 Figure 6-23: BMP RT.07 - Media Filter Drain Type 1 2
6 - 86 August 2010
1 Figure 6-24: BMP RT.07 - Media Filter Drain Type 2 2
6 - 87 August 2010
1 Figure 6-25: BMP RT.07 - Media Filter Drain Type 3 2
6 - 88 August 2010
1
2
Figure 6-26: BMP RT.07 - Media Filter Drain Type 4 3
4
6 - 89 August 2010
Applications and Limitations 1
Applications 2
Media Filter Drains 3
The MFD can achieve basic, phosphorus, and enhanced water quality treatment. 4
Because maintaining sheet flow across the MFD is required for its proper 5
function, the ideal locations for MFDs in highway settings are highway side 6
slopes or other long, linear grades with lateral side slopes less than 4H:1V and 7
longitudinal slopes no steeper than 5 percent. As side slopes approach 3H:1V, 8
without design modifications, sloughing may become a problem due to friction 9
limitations between the separation geotextile and underlying soils. The longest 10
flow path from the contributing area delivering sheet flow to the MFD should not 11
exceed 150 ft. 12
Four typical applications are shown in Figures 6-23 through 6-26. Figure 6-23 13
(Type 1) shows the typical installation for roadways that have a roadside ditch for 14
stormwater conveyance. The design stormwater level in the ditch should be 15
below the media filter mix; otherwise, erosion of the mix and flushing of the 16
trapped contaminates may occur. 17
Figure 6-24 (Type 2) shows the typical application where flow from the media 18
filter mix needs to be collected and transported by perforated pipe to a separate 19
storm drain, outfall, or detention facility. 20
Figure 6-25 (Type 3) is where a MFD needs to be installed on an existing side 21
slope that is less than 4H:1V. Figure 6-26 (Type 4) is the typical situation for a 22
roadway in a cut slope condition, where there is no room for a conventional edge 23
of road drainage ditch. For this condition, the underdrain must be sized for not 24
only the roadway runoff, but the back slope and any off-site runoff as well. 25
Note that the underdrain section can be sized to act as an infiltration trench for 26
the Types 2, 3 and 4 media filter drains. The underlying infiltration trench would 27
be sized to detain the design stormwater volume until it infiltrates into the 28
underlying soil or limestone bedrock strata. Where infiltration capacity is not 29
sufficient to fully infiltrate the design volume, the perforated underdrain pipe can 30
be utilized as a conveyance to a separate overflow type discharge or detention 31
location. 32
Limitations 33
Media Filter Drains 34
Steep slopes. Avoid construction on longitudinal slopes steeper than 5 percent. 35
Avoid construction on steeper than 3H:1V lateral slopes, and preferably use 36
4H:1V slopes or less. In areas where lateral slopes exceed 4H:1V, it may be 37
6 - 90 August 2010
possible to construct terraces to create 4H:1V slopes or to otherwise stabilize up 1
to 3H:1V slopes. (For details, see ―Geometry, Components, and Sizing Criteria, 2
Cross Section in the Structural Design Considerations‖ section, below). 3
Wetlands. Do not construct in wetlands and wetland buffers. In many cases, an 4
MFD (due to its small lateral footprint) can fit within the highway fill slopes 5
adjacent to a wetland buffer. In those situations where the highway fill prism is 6
located adjacent to wetlands, an interception trench/underdrain will need to be 7
incorporated as a design element in the MFD. 8
Shallow groundwater. Mean high-water table levels at the project site need to 9
be determined to ensure the MFD mix bed and the underdrain, if needed, will not 10
become saturated by shallow groundwater. 11
Unstable slopes. In areas where slope stability may be problematic, consult a 12
Geotechnical Engineer. 13
Design Flow Elements 14
Flows To Be Treated 15
The basic design concept behind the MFD is to fully filter all the roadway runoff 16
through the MFD mix. Therefore, the infiltration capacity of the medium and drainage 17
below needs to match or exceed the design frequency storm hydraulic loading rate. 18
Where the MFD underdrain is acting as the roadway collection conveyance, then the 19
infiltration and underdrain capacity must meet the appropriate design storm 20
frequency (see Chapter 5). 21
Structural Design Considerations 22
Geometry 23
No-Vegetation Zone 24
The no-vegetation zone (vegetation-free zone) is a shallow, gravel trench located 25
directly adjacent to the highway pavement. The no-vegetation zone is a crucial 26
element in a properly-functioning MFD or other BMPs that use sheet flow to 27
convey runoff from the highway surface to the BMP. The no-vegetation zone 28
functions as a level spreader to promote sheet flow and as a deposition area for 29
coarse sediments. The no-vegetation zone should be between 1 ft. and 3 ft. 30
wide. Depth will be a function of how the roadway section is built from sub-grade 31
to finished grade; the resultant cross section will typically be triangular to 32
trapezoidal. 33
Grass Strip 34
The width of the grass strip is dependent on the availability of space within the 35
highway side slope. The baseline design criterion for the grass strip within the 36
6 - 91 August 2010
MFD is a 3-ft. minimum width, but wider grass strips are recommended if 1
additional space is available. The designer should bring in a suitable topsoil or 2
native soil with at least 2 in. of compost tilled in. The designer may consider 3
adding aggregate to the soil mix to help minimize rutting problems from errant 4
vehicles. The soil mix should ensure grass growth for the design life of the MFD. 5
The grass seed mix should be a seed mix as recommended by the NRCS or 6
DAWR for the site location and the seasonal conditions at the time of planting. 7
Media Filter Drain Mix Bed 8
The MFD mix is a mixture of crushed rock (screened to 3/8 in. to #10 sieve), 9
dolomite, gypsum, and perlite (see Table 6-6). The crushed rock provides the 10
support matrix of the medium; the dolomite and gypsum add alkalinity and ion 11
exchange capacity to promote the precipitation and exchange of heavy metals; 12
and the perlite improves moisture retention to promote the formation of biomass 13
within the MFD mix. The combination of physical filtering, precipitation, ion 14
exchange, and biofiltration enhances the water treatment capacity of the mix. 15
The MFD mix has an estimated initial filtration rate of 50 in./hour and a long-term 16
filtration rate of 28 in./hour, due to siltation. With an additional safety factor, the 17
rate used to size the length of the MFD should be 10 in./hour. 18
19
6 - 92 August 2010
Media Filter Drain Materials Quantity
Mineral Aggregate: Crushed screenings 3/8-in. to #10 sieve
Crushed screenings should be manufactured from ledge rock, talus, or gravel
in accordance with Section 3-01 of the Standard Specifications for Road,
Bridge, and Municipal Construction (2002), which meets the following test
requirements:
Los Angeles Wear, 500 Revolutions 35% max. Degradation Factor 30 min.
Crushed screenings should conform to the following requirements for grading
and quality:
Sieve Size Percent Passing (by weight)
1/2 in. square 100
3/8 in. square 90-100
U.S. No. 4 30-56
U.S. No. 10 0-10
U.S. No. 200 0-1.5
% fracture, by weight, min. 75
Static stripping test Pass
The fracture requirement should be at least one fractured face and will apply to
material retained on the U.S. No. 10 if that sieve retains more than 5% of the
total sample.
The finished product should be clean; uniform in quality; and free from wood,
bark, roots, and other deleterious materials.
Crushed screenings should be substantially free from adherent coatings. The
presence of a thin, firmly adhering film of weathered rock should not be
considered as coating unless it exists on more than 50 percent of the surface
area of any size between successive laboratory sieves.
3 cubic yards
Perlite:
Horticultural grade, free of any toxic materials
0-30% passing U.S. No. 18 Sieve
0-10% passing U.S. No. 30 Sieve
1 cubic yard per
3 cubic yards of
mineral
aggregate
Dolomite: CaMg(CO3)2 (calcium magnesium carbonate)
Agricultural grade, free of any toxic materials
100% passing U.S. No. 8 Sieve
10 pounds per
cubic yard of
perlite
6 - 93 August 2010
Media Filter Drain Materials Quantity
0% passing U.S. No. 16 Sieve
Gypsum: Non-calcined, agricultural gypsum CaSO4•2H2O (hydrated
calcium sulfate)
Agricultural grade, free of any toxic materials
100% passing U.S. No. 8 Sieve
0% passing U.S. No. 16 Sieve
1.5 pounds per
cubic yard of
perlite
Table 6-6: Media Filter Drain Mix 1
Conveyance System Below Media Filter Drain Mix 2
The gravel under-layer and/or underdrain trench provides hydraulic conveyance. 3
The underdrain is used when treated runoff needs to be conveyed to a desired 4
location, such as a downstream flow control facility or stormwater outfall. The 5
underdrain trench should be a minimum of 2 ft. wide. In the limestone bedrock 6
areas of northern Guam, the underdrain should be designed as an infiltration 7
trench sized to detain and infiltrate the design storm event. In the case where 8
there is no overtopping discharge location, then the under drain infiltration trench 9
should be sized to detain and infiltrate the 100-year storm. 10
The gravel underdrain trench may be eliminated if there is evidence to support 11
that flows can be conveyed laterally to an adjacent ditch or onto a fill slope that is 12
properly vegetated to protect against erosion. The MFD mix should be kept free 13
to drain, or un-submerged above the 25-year storm event water surface 14
elevations in the downstream ditch. 15
Sizing Criteria 16
Width 17
The width of the MFD mix bed is determined by the amount of contributing 18
pavement routed to the embankment. The surface area of the MFD mix bed 19
needs to be sufficiently large to fully infiltrate the runoff treatment design flow rate 20
using the long-term filtration rate of the MFD mix. For design purposes, a 50 21
percent safety factor is incorporated into the long-term MFD mix filtration rate to 22
accommodate variations in slope, resulting in a design filtration rate of 10 in. per 23
hour. The MFD mix bed should have a bottom width of at least 2 ft. in contact 24
with the conveyance system below the MFD mix. 25
Length 26
In general, the length of an MFD is the same as the contributing pavement. Any 27
length is acceptable as long as the surface area MFD mix bed is sufficient to fully 28
infiltrate the stormwater quality treatment design flow rate. 29
6 - 94 August 2010
Cross-Section 1
In profile, the surface of the MFD should preferably have a lateral slope less than 2
4H:1V (<25 percent). On steeper terrain, it may be possible to construct terraces 3
to create a 4H:1V slope, or other engineering may be employed to ensure a 4
slope stability up to 3H:1V. If sloughing is a concern on steeper slopes, 5
consideration should be given to incorporating permeable soil reinforcements 6
such as geotextiles, open-graded/permeable pavements, or commercially-7
available ring and grid reinforcement structures, as top layer components to the 8
MFD mix bed. Consultation with a Geotechnical Engineer is required. 9
Inflow 10
Runoff is conveyed to an MFD using sheet flow from the pavement area. The 11
longitudinal pavement slope contributing flow to an MFD should be less than 12
5 percent. Although there is no lateral pavement slope restriction for flows going 13
to an MFD, the designer should ensure flows remain as sheet flow. 14
Media Filter Drain Mix Bed Sizing Procedure 15
The MFD mix should be a minimum of 12 in. deep, including the section on top of the 16
underdrain trench. 17
For runoff treatment, sizing the MFD mix bed is based on the requirement that the 18
runoff treatment flow rate from the pavement area, QHighway, cannot exceed the long-19
term infiltration capacity of the MFD, QInfiltration: 20
21
22
Calculations should be based on a long-term infiltration capacity of 10 in. per hour. 23
From a practical standpoint, it has been found that the media filter drain should have 24
the minimum widths shown in Table 6-7. 25
26
Pavement width that contributes runoff to the media filter drain
Minimum media filter drain width*
< 20 ft. 2 ft.
> 20 and < 35 ft. 3 ft.
> 35 ft. 4 ft.
Table 6-7: Media Filter Drain Widths 27
*Width does not include the required 1-ft. to 3-ft. gravel vegetation-free zone or the 3-ft. filter strip 28 width. 29
6 - 95 August 2010
Materials 1
Media Filter Drain Mix 2
The MFD mix used in the construction of MFDs consists of the material proportions 3
listed in Table 6-6. Mixing and transportation must occur in a manner that ensures 4
the materials are thoroughly mixed prior to placement, and that separation does not 5
occur during transportation or construction operations. 6
Site Design Elements 7
Planting Considerations 8
Planting of the grassed filter strip is the same as for biofiltration swales. 9
Operations and Maintenance 10
Maintenance will consist of routine roadside management. While herbicides will not 11
be applied directly over the MFD, it may be necessary to periodically control noxious 12
weeds with herbicides in areas around the MFD. 13
The design life of the MFD is expected to be approximately 20 years. At that time, 14
the MFD mix should be removed and replaced with new filter mix. The mix 15
replacement should normally be part of any adjacent pavement rehabilitation or 16
resurfacing project. 17
Signing 18
Non-reflective guideposts should delineate the MFD. This practice allows DPW‘s 19
personnel to identify where the system is installed and to make appropriate repairs 20
should damage occur to the system. 21
6.4.4 WET POOL: BMPS 22
Wet pool BMPs include the following: 23
RT.08 – Wet Pond 24
RT.10 – Constructed Stormwater Treatment Wetland 25
RT.09 – Combined Wet/Detention Pond 26
RT.11 – Combined Stormwater Treatment Wetland/Detention Pond 27
Wet pool BMPs are constructed basins containing a permanent pool of water. Wet 28
ponds function by settling suspended solids. The biological action of plants and bacteria 29
provides some additional treatment. Not only can wet ponds be designed for the 30
treatment of conventional pollutants, but they can also be modified (such as a created 31
wetland) to enhance the removal of nutrients or dissolved metals. Wet ponds are usually 32
more effective and efficient when constructed using multiple cells (a series of individual 33
smaller basins), where coarser sediments become trapped in the first cell, or forebay. 34
6 - 96 August 2010
Wet pond designs can also provide flow control by adding detention volume (live 1
storage) above the dead storage. 2
A wet pool BMP can be either an on- or off-line facility receiving the stormwater quality 3
treatment volume. 4
Constructed stormwater quality treatment wetlands offer a suitable alternative to wet 5
ponds or biofiltration swales and can also provide treatment for dissolved metals. 6
However, designers must consider the availability of water and the water needs of plants 7
used in the stormwater wetland. The landscape context for stormwater wetland 8
placement must be appropriate for the creation of an artificial wetland (groundwater, 9
soils, and surrounding vegetation). Natural wetlands cannot be used for stormwater 10
treatment purposes. 11
6.4.4.1 RT.08: WET POND 12
General Description 13
A wet pond is a constructed stormwater pond that retains a permanent pool of water 14
(wet pool), at least during the wet season. The volume of the wet pool is related to the 15
effectiveness of the pond in settling particulate pollutants. As an option, a shallow marsh 16
area can be created within the permanent pool volume to provide additional treatment 17
for nutrient removal. The BMP RT.09 Combined Wet/Detention Pond is the wet pond 18
provided with live storage area above the permanent pool for detention flow control. 19
Figures 6-27 and 6-28 illustrate a typical wet pond BMP. 20
Applications and Limitations 21
Wet ponds can be designed in two sizes -- basic and large. A basic wet pond is the 22
stormwater quality treatment BMP for the usual case. Large wet ponds are designed for 23
higher levels of pollutant removal and are an appropriate treatment BMP for phosphorus 24
removal. In areas of higher soil permeability, wet ponds should be lined with 25
impermeable membranes. 26
Refer to RT.09-Combined Wet/Detention Pond if the pond is to be used for flow control 27
in addition to runoff treatment. 28
Design Flow Elements 29
Flows To Be Treated 30
Basic wet ponds are designed to treat the runoff treatment volume described as part 31
of Minimum Requirement 5 as defined in Chapter 3 of this TSDM. Large wet ponds 32
are designed to treat a volume 1.5 times greater than the water quality treatment 33
volume. 34
35
36
6 - 97 August 2010
Overflow or Bypass 1
The overflow criteria for single-purpose (treatment only, not combined with flow 2
control) wet ponds are as follows: 3
An open-top, standpipe riser in the control structure satisfies the requirement 4
for primary overflow design. 5
The top of the riser should be set at the water quality design water surface 6
elevation. 7
If it is an on-line pond, the primary overflow should be sized to pass the 100-8
year flow. If the pond is off-line (upstream flow splitter to bypass the flows in 9
excess of the design stormwater quality treatment flow), the primary overflow 10
should be sized to pass the upstream conveyance capacity. 11
Emergency Overflow Spillway 12
An emergency spillway or structure must be provided and designed according to the 13
requirements for BMP FC.01-Detention Pond. The control surface of the emergency 14
overflow spillway must be set at the design water surface elevation. 15
Structural Design Considerations 16
Geometry 17
Design Criteria 18
The wet pond is divided into a minimum of two cells, separated by a baffle or 19
berm. The first cell must contain between 25 percent and 35 percent of the total 20
wet pond volume. The baffle or berm volume does not count as part of the total 21
wet pond volume. The term baffle means a vertical divider placed across the 22
entire width of the pond, stopping short of the bottom. A berm is a vertical divider 23
typically built up from the bottom; in a wet vault, it connects all the way to the 24
bottom. 25
Intent: The full-length berm or baffle promotes plug flow and enhances 26
quiescence and laminar flow through as much of the entire water volume as 27
possible. Alternative methods to the full-length berm or baffle that provide 28
equivalent flow characteristics may be approved on a case-by-case basis by 29
DPW. 30
Sediment storage is provided in the first cell. The minimum depth of the sediment 31
storage must be 1 ft. A fixed sediment depth monitor should be installed in the 32
first cell to gauge sediment accumulation, or an alternative gauging method 33
should be used. 34
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The minimum depth of the first cell must be 4 ft., exclusive of sediment storage 1
requirements. The depth of the first cell may be greater than the depth of the 2
second cell. 3
The maximum depth of each cell must not exceed 8 ft. (exclusive of sediment 4
storage in the first cell). Pool depths of 3 ft. or shallower (second cell) must be 5
planted with emergent wetland vegetation (see ―Landscaping‖ later in this 6
section). 7
Inlets and outlets must be placed to maximize the flow path through the facility. 8
The ratio of flow path length to width from the inlet to the outlet must be at least 9
2:1. The flow path length is defined as the distance from the inlet to the outlet, as 10
measured at mid-depth. The width at mid-depth is calculated as follows: width = 11
(average top width + average bottom width)/2. 12
Wet ponds with wet pool volumes less than or equal to 4,000 cu. ft. may be 13
single-celled (no baffle or berm is required). However, it is especially important 14
that the flow path length be maximized in single-celled wet ponds. The ratio of 15
flow path length to width must be greater than 4:1 in single-celled wet ponds. 16
All inlets must enter the first cell. If there are multiple inlets, the length-to-width 17
ratio is based on the average flow path length for all inlets. 18
Sizing Procedure 19
Design Steps (D) 20
D-1 Identify the required wet pool volume (Volwq). For large wet ponds, 21
the wet pool volume is 1.5 times the water quality volume. 22
D-2 Estimate wet pool dimensions satisfying the following design 23
criterion: 24
Volwq = [h1(At1 + Ab1) / 2] + [h2(At2 + Ab2) / 2] +……+ [hn(Atn 25
+ Abn) / 2] 26
Where: 27
Atn = Top area of wet pool surface in cell n in ft.2 28
Abn = Bottom area of wet pool surface in cell n in ft.2 29
hn = Depth of wet pool in cell n (above top of sediment 30
storage) in ft. 31
D-3 Design pond outlet pipe and determine primary overflow water 32
surface 33
34
35
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Inlet and Outlet 1
For details on the following requirements, see Figures 6-27 and 6-28. 2
The inlet to the wet pond must be submerged, with the inlet pipe invert a minimum of 3
2 ft. from the pond bottom (not including the 1-ft. minimum sediment storage). The top of 4
the inlet pipe should be submerged at least 1 ft. below the runoff treatment design water 5
surface, if possible. The designer should compute the Hydraulic Grade Line (HGL) of the 6
inlet pipe to verify that backwater conditions are acceptable. 7
Intent: The inlet is submerged to dissipate the energy of the incoming flow. The distance 8
from the bottom is set to minimize the re-suspension of settled sediments. Alternative 9
inlet designs that accomplish these objectives are acceptable. 10
An outlet structure must be provided. No sump is required in the outlet structure for wet 11
ponds not providing detention storage. The outlet structure receives flow from the pond 12
outlet pipe. The birdcage opening provides an overflow route should the pond outlet pipe 13
become clogged. 14
The pond outlet pipe (from the pond into the outlet structure) must be back-sloped, or 15
have a turn-down elbow, and extend 1 ft. below the runoff treatment design water 16
surface. A floating outlet, set to draw water from 1 ft. below the water surface, is also 17
acceptable if vandalism concerns are adequately addressed. 18
Intent: The inverted outlet pipe traps oils and floatables in the wet pond. 19
The pond outlet pipe must be sized, at a minimum, to pass the runoff treatment design 20
flow. Note: The highest invert of the outlet pipe sets the runoff treatment design water 21
surface elevation. 22
Materials 23
All metal parts must be corrosion-resistant. Galvanized materials should not be used 24
unless unavoidable. 25
Intent: Galvanized metal contributes zinc to stormwater, sometimes in very high 26
concentrations. 27
Berms, Baffles, and Slopes 28
A berm or baffle must extend across the full width of the wet pool and tie into the wet 29
pond side slopes. If the berm embankments are greater than 4 ft. high, the berm must be 30
constructed by excavating a key trench equal to 50 percent of the embankment cross-31
sectional height and width. A Geotechnical Engineer may waive this requirement for 32
specific site conditions. A geotechnical analysis must address situations in which one of 33
the two cells is empty while the other remains full of water. 34
The top of the berm should be submerged 1 ft. below this surface. Earthen berms should 35
have a minimum top width of 5 ft. 36
6 - 100 August 2010
Intent: The submerged berm of 1 ft. provides a surface skimming effect to improve the 1
sedimentation settlement efficiency. 2
If good vegetation cover is not established on the berm, erosion control measures 3
should be used to prevent erosion of the berm back-slope when the pond is initially filled. 4
Instead of an interior berm, a structural type retaining wall may be used. 5
The criteria for wet pond side slopes are as follows: 6
Interior side slopes should be no steeper than 2H:1V. However, slopes of 3H:1V 7
or flatter are preferred. If using 2H:1V slopes, the pond must be fenced for safety 8
reasons. 9
Exterior side slopes must be no steeper than 2H:1V. 10
Slopes should be no steeper than 4H:1V if they are to be mowed. 11
Pond sides may be retaining walls, provided that a fence is situated along the top 12
of the wall and at least 25 percent of the pond perimeter is a vegetated side 13
slope no steeper than 3H: 1V. 14
The toe of the exterior slope must be no closer than 5 ft. from the right-of-way 15
line. 16
Embankments 17
The berm embankment must be constructed of material consisting of a minimum of 30 18
percent clay, a maximum of 60 percent sand, a maximum of 60 percent silt, and 19
negligible gravel and cobble. 20
To prevent undermining, installation of a perimeter cutoff trench underneath or near 21
embankments should be considered. 22
Anti-seepage collars must be placed on outflow pipes in berm embankments impounding 23
water deeper than 8 ft. at the runoff treatment design water surface. Anti-seepage collars 24
may also be necessary in other situations. 25
Ponds having storage impoundments of 10 ac-ft. or greater, must have DPW‘s pre-26
approval, where the design of the pond should meet federal or typical industry large dam 27
safety guidelines in regards to the hydrology, geotechnical, structural, and hydraulic 28
considerations. 29
Site Design Elements 30
Setback Requirements 31
Wet ponds must be a minimum of 5 ft. from any property line or vegetative buffer, 32
and must be 100 ft. from any septic tank or drain field. 33
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The designer should request a geotechnical report for the project that evaluates any 1
potential structural site instability due to extended sub-grade saturation or head-2
loading of the permeable layer. This includes the potential impacts to down-gradient 3
properties, especially on hills with known side-hill seeps. The report should address 4
the adequacy of the proposed wet pond locations and recommend the necessary 5
setbacks from any steep slopes and building foundations. 6
Landscaping (Planting Considerations) 7
Planting requirements for detention ponds also apply to wet ponds. 8
The cells in large wet ponds intended for phosphorus control should not be planted 9
because the plants release phosphorus in the winter when they die off. 10
If the second cell of a basic wet pond is 3 ft. or shallower, the bottom area must be 11
planted with emergent wetland vegetation. This results in habitat for natural 12
predators of mosquitoes such as dragonflies, birds, fish, and frogs. 13
Intent: The planting of shallow pond areas helps to stabilize settled sediment and 14
prevent re-suspension. 15
Vegetation that forms floating mats should not be planted because the mats protect 16
mosquito larvae. 17
A variety of plant species should be planted to encourage a diversity of mosquito 18
predators. This variety can also make the overall predator population more robust to 19
withstand environmental changes to the facility. 20
If the wet pond discharges to a phosphorus-sensitive lake or wetland, shrubs that 21
form a dense cover should be planted on slopes above the runoff treatment design 22
water surface on at least three sides. For banks that are berms, no planting is 23
allowed if the berm is regulated by dam safety requirements. The purpose of planting 24
is to provide shading. 25
Fencing 26
Pond walls may be retaining walls as long as a fence is provided along the top of the 27
wall and at least 25 percent of the pond perimeter will have a slope of 3H:1V or 28
flatter. For safety reasons, the pond must be fenced if the slopes are steeper than 29
3H:1V. 30
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1 Figure 6-27: BMP RT.08 - Wet Pond 2
6 - 103 August 2010
1 Figure 6-28: Wet Pond Cross Sections 2
Recommended Design Features 3
The following design features should be incorporated into the wet pond design where 4
site conditions allow: 5
For wet pool depths in excess of 6 ft., it is recommended that some form of recirculation 6
be provided. A fountain or aerator may be used to prevent stagnation and low-dissolved 7
6 - 104 August 2010
oxygen conditions. Alternatively, a small amount of base flow could be directed to the 1
pond to maintain circulation and reduce the potential for low-oxygen conditions. 2
Trees should be planted along the west and south sides of ponds to reduce thermal 3
heating, except that no trees or shrubs may be planted on compacted fill berms. Trees 4
should be set back so that the branches will not extend over the pond. 5
The number of inlets to the facility should be limited; ideally, there should be only one 6
inlet. Regardless of the number of inlets, the flow path length between the inlet and 7
outlet should be maximized. 8
Where possible, the following design features should be incorporated to enhance 9
aesthetics: 10
Provide side slopes that are sufficiently gentle (3H:1V or flatter) to avoid the need 11
for fencing. Gentler slopes typically allow a facility to better blend into its 12
surroundings. 13
Use sinuous or irregularly-shaped ponds to create a more naturalistic landscape. 14
Provide visual enhancement with clusters of trees and shrubs. On most pond 15
sites, it is important to amend the soil before planting because ponds are typically 16
placed well below the native soil horizon in very poor soils. Make sure that dam 17
safety restrictions against planting do not apply. 18
Orient the pond length along the direction of prevailing summer winds to enhance 19
wind mixing. 20
Impermeable liners may be necessary to maintain the wet pool volume in 21
permeable soils. 22
Construction Criteria 23
If the pond has been used for temporary sedimentation purposes during construction, 24
the accumulated sediment must be cleaned out at the end of construction. 25
6.4.4.2 RT.10: COMBINED WET/DETENTION POND 26
General Description 27
A combined detention and runoff treatment wet pond facility has the appearance of a 28
detention facility but contains a permanent pool of water as well. The following design 29
procedures, requirements, and recommendations cover differences in the design of the 30
stand-alone runoff treatment facility when combined with detention storage. 31
There are two sizes of the combined wet pond -- basic and large. The facility sizes 32
(basic and large) are related to the pollutant-removal goals. 33
34
35
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Applications and Limitations 1
Combined detention and runoff treatment facilities are very efficient for sites that also 2
have flow control requirements, but are not conducive to dispersion or infiltration. The 3
runoff treatment BMP may often be placed beneath detention storage without increasing 4
the overall facility surface area. However, the fluctuating water surface of the live storage 5
creates unique challenges for plant growth and aesthetics. 6
The basis for pollutant removal in combined facilities is the same as that for stand-alone 7
runoff treatment facilities. However, in the combined facility, the detention function 8
creates fluctuating water levels and added turbulence. For simplicity, the positive effect 9
of the extra live storage volume and the negative effect of increased turbulence are 10
assumed to balance, and are thus ignored when the wet pool volume is sized. For the 11
combined detention/stormwater wetland, criteria that limit the extent of water level 12
fluctuation are specified to better ensure survival of the wetland plants. 13
Unlike the wet pool volume, the live storage component of the facility must be provided 14
above the seasonal high-water table. It is recommended that all stormwater quality 15
treatment BMPs that use permanent wet pools use facility liners. 16
Typical design details and concepts for a combined detention and wet pond are shown 17
in Figures 6-29 and 6-30. The detention portion of the facility must meet the design 18
criteria and sizing procedures set forth in the flow control BMP FC.01 - Detention Pond. 19
Design Flow Elements 20
Flows To Be Treated 21
Basic combined wet/detention ponds are designed to treat the runoff treatment 22
volume and detain flows according to the criteria described as part of Minimum 23
Requirements 5 and 6, respectively, as defined in Chapter 3 of this TSDM. Large 24
combined wet/detention ponds are designed to treat 1.5 times the runoff treatment 25
volume. 26
Structural Design Considerations 27
Geometry 28
The geometry criteria for wet ponds (see BMP RT.08) apply, with the following 29
modifications and clarifications: 30
The permanent pool may be made shallower to take up most of the pond bottom, or 31
it may be deeper and positioned to take up only a limited portion of the bottom. Wet 32
pond criteria governing water depth, however, must still be met. (See Figure 6-31 for 33
alternate possibilities for wet pool cell placement.) 34
Intent: This flexibility in positioning cells allows for multiple-use options in live storage 35
areas during the drier months. 36
6 - 106 August 2010
The minimum sediment storage depth in the first cell is 1 ft. The 6 in. of sediment 1
storage required for a detention pond does not need to be added to this, but 6 in. of 2
sediment storage must be added to the second cell to comply with the detention 3
sediment storage requirement. 4
The wet pool and sediment storage volumes are not included in the required 5
detention volume. 6
Sizing Procedure 7
The sizing procedure for combined detention and wet ponds is identical to that 8
outlined for wet ponds (see BMP RT.08) and detention ponds (see BMP FC.01). 9
Inlet and Outlet 10
The inlet and outlet criteria for wet ponds (see BMP RT.08) apply, with the following 11
modifications: 12
A sump must be provided in the outlet control structure of combined ponds 13
The detention flow restrictor and its outlet pipe must be designed according to 14
the requirements for detention ponds (see BMP FC.01) 15
Berms, Baffles, and Slopes 16
The criteria are the same as for wet ponds (see BMP RT.08). 17
Groundwater Issues 18
Live storage requirements are the same as for detention ponds (see BMP FC.01). 19
This does not apply to the wet pond dead storage component. 20
Site Design Elements 21
General maintenance requirements and setback criteria are the same as for wet 22
ponds (see BMP RT.08). 23
Planting Requirements 24
The criteria are the same as for wet ponds (see BMP RT.08). 25
26
6 - 107 August 2010
1 Figure 6-29: BMP RT.10 - Combined Detention and Wet Pond – Plan View 2
6 - 108 August 2010
1
2
3 Figure 6-30: Combined Detention and Wet Pond – Cross Sections 4
5
6
6 - 109 August 2010
1 Figure 6-31: Alternative Configuration of Detention and Wet Pond 2
6.4.4.3 RT.09: CONSTRUCTED STORMWATER TREATMENT 3
WETLAND 4
General Description 5
Stormwater treatment wetlands are shallow constructed wetlands designed to treat 6
stormwater through settling, filtering, and the biological processes associated with 7
emergent aquatic plants. Stormwater treatment wetlands, like wet ponds, are used to 8
capture and transform pollutants. Over time, these pollutants concentrate in the 9
sediment. 10
6 - 110 August 2010
Instead of treating stormwater runoff, some wetlands are constructed to replace or 1
mitigate impacts when natural wetlands are filled or impacted by development (mitigation 2
wetlands). Natural wetlands and mitigation wetlands cannot be used to treat stormwater. 3
Applications and Limitations 4
As an enhanced treatment BMP, stormwater wetlands can be considered for roadways 5
where metal removal is a concern. Stormwater wetlands occupy roughly the same 6
surface area as wet ponds, but they have the potential to be better aesthetically 7
integrated into a site because of the abundance of emergent aquatic vegetation. The 8
most critical factor for a successful design is an adequate supply of water for most of the 9
year. Careful planning is needed to ensure sufficient water is retained to sustain good 10
wetland plant growth. Because water depths in stormwater wetlands are shallower than 11
in wet ponds, water loss by evaporation is an important concern. Stormwater wetlands 12
are a good runoff treatment facility choice in areas where groundwater levels are high in 13
the wet season. It is recommended that all runoff treatment BMPs that use permanent 14
wet pools use facility liners. 15
Design Flow Elements 16
Flows To Be Treated 17
Constructed stormwater treatment wetlands are designed for the stormwater quality 18
treatment volume (VolWQ) described as part of Minimum Requirement 5 as defined in 19
Chapter 3 of this TSDM. 20
Overflow or Bypass 21
The overflow criteria for single-purpose wetlands (treatment only, not combined with 22
flow control) follow the same criteria for wet ponds (see BMP RT.08). 23
Emergency Overflow Spillway 24
An emergency spillway must be provided and designed according to the 25
requirements for detention ponds (see BMP FC.01). In addition, a bypass or shutoff 26
valve to enable the wetland to be taken off-line for maintenance purposes should be 27
provided, if possible. 28
Bioengineered stabilization measures should be provided at the end of the outlet 29
pipe and spillway to minimize the need for riprap and to increase aesthetics. 30
Structural Design Considerations 31
Geometry 32
Design Criteria 33
1. Stormwater wetlands must consist of two cells -- a pre-settling cell and a 34
wetland cell. 35
6 - 111 August 2010
2. The pre-settling cell must contain approximately 33 percent of the wet 1
pool volume. 2
3. The depth of the pre-settling cell must be between 4 ft. (minimum) and 3
8 ft. (maximum), excluding sediment storage. 4
4. The pre-settling cell must provide 1 ft. of sediment storage. 5
5. The wetland cell must have an average water depth of approximately 6
1.5 ft. (plus or minus 3 in.). 7
Sizing Procedure 8
Step 1. Specify the depth of the pre-settling cell (Dpc ft.). See Criterion 3 in 9
―Design Criteria,‖ listed above. 10
Step 2. Determine the volume of the pre-settling cell (Vpc ft.3) by using Criterion 2 11
in ―Design Criteria,‖ listed above. 12
Step 3. Determine the surface area of the pre-settling cell (Apc ft.2) of the 13
stormwater wetland using the pre-settling cell volume and depth. Apc = Vpc / Dpc. 14
Step 4. Calculate the surface area of the stormwater wetland. The surface area 15
of the entire wetland (Atotal ft.2) must be the same as the top area of a wet pond 16
sized for the same site conditions. The surface area of the entire stormwater 17
wetland is the runoff treatment wet pool volume divided by the wet pool water 18
depth (use 3 ft.). 19
Atotal = Vtotal / 3 ft. 20
Step 5. Determine the surface area of the wetland cell (Awc ft.2). Subtract the 21
surface area of the pre-settling cell from the total wetland surface area. 22
(Atotal). Awc = Atotal – Apc. 23
Step 6. Determine water depth distributions in the wetland cell. Decide whether 24
the top of the dividing berm is at the surface or submerged (designer's choice). 25
Adjust the distribution of water depths in the wetland cell according to the surface 26
area of the wetland cell (Awc) and the discussion below. Note: This results in a 27
facility that holds less volume than the runoff treatment volume; which is 28
acceptable. 29
Intent: The surface area of the stormwater wetland is set to be roughly equivalent to 30
that of a wet pond (with an average depth of the wetland cell of 1.5 ft.) designed for 31
the same site so as not to discourage use of this option. 32
Two examples for grading the bottom of the wetland cell are shown in Figures 6-32 33
and 6-33. Option A is a shallow, evenly-graded slope from the upstream to the 34
downstream edge of the wetland cell. The second example, Option B, is a 35
naturalistic alternative with the specified range of depths intermixed throughout the 36
6 - 112 August 2010
second cell. A distribution of depths must be provided in the wetland cell depending 1
on whether the dividing berm is at the water surface or submerged (see Table 6-8). 2
The maximum depth is 2.5 ft. in either configuration. 3
4
Dividing Berm at Runoff Treatment Design Water Surface Dividing Berm Submerged 1 Foot
Depth Range (feet) Percent Depth Range (feet) Percent
0.1 to 1 25 1 to 1.5 40
1 to 2 55 1.5 to 2 40
2 to 2.5 20 2 to 2.5 20
Table 6-8: Distribution of Depths in Wetland Cell 5
Inlet 6
The inlet to the pre-settling cell of the wetland must be submerged, with the inlet pipe 7
invert a minimum of 2 ft. from the wetland bottom, not including sediment storage. The 8
top of the inlet pipe should be submerged at least 1 ft., if possible. The designer should 9
compute the HGL of the inlet pipe to verify whether backwater conditions are acceptable. 10
Intent: The inlet is submerged to dissipate the energy of the incoming flow. The distance 11
from the bottom is set to minimize the re-suspension of settled sediments. Alternative 12
inlet designs that accomplish these objectives are acceptable. 13
Outlet 14
An outlet structure must be provided. No sump is required in the outlet structure for 15
wetlands not providing detention storage. The outlet structure receives flow from the 16
wetland outlet pipe. The birdcage opening provides an overflow route should the wetland 17
outlet pipe become clogged. The following overflow criteria specify the sizing and 18
position of the grate opening: 19
The wetland outlet pipe (from the wetland into the control structure) must be 20
back-sloped or have a turn-down elbow and extend 1 ft. below the runoff 21
treatment design water surface. 22
Intent: The inverted outlet pipe traps oils and floatables in the wetland. 23
At a minimum, the wetland outlet pipe must be sized to pass the runoff treatment 24
design flow for off-line wetlands. For on-line wetlands, the outlet pipe must be 25
able to pass the maximum possible inlet conveyance flow. Note: The highest 26
invert of the outlet pipe sets the runoff treatment design water surface elevation. 27
Alternative methods to dissipate energy at the end of the outlet pipe, such as a 28
dissipater tee, should be considered to reduce the need for extensive riprap. 29
6 - 113 August 2010
Berms, Baffles, and Slopes 1
The berm separating the two cells must be shaped so that its downstream side gradually 2
slopes to form the second shallow wetland cell (see the section view in Figure 6-31). 3
Alternately, the second cell may be graded naturalistically from the top of the dividing 4
berm (see ―Sizing Procedure,‖ Step 6, above). 5
The top of the berm must be either at the runoff treatment design water surface or 6
submerged 1 ft. below this surface, as for wet ponds. 7
Correspondingly, the side slopes of the berm must meet the following criteria: 8
If the top of the berm is at the runoff treatment design water surface, then for 9
safety reasons, the berm should not be greater than 3H:1V, just as the wetland 10
banks should not be greater than 3H:1V if the wetland is not fenced. 11
If the top of the berm is submerged 1 ft., the upstream side slope may be up to 12
2H:1V. If submerged, the berm is not considered accessible, and the steeper 13
slope is allowable. 14
Liners 15
If soil permeability allows for sufficient water retention, lining is not necessary. In 16
infiltrative soils, both cells of the stormwater wetland must be lined with an impermeable 17
membrane. 18
Site Design Elements 19
Setback Requirements 20
Stormwater treatment wetlands must be a minimum of 5 ft. from any property line or 21
vegetative buffer. 22
Unlined stormwater treatment wetlands must be 100 ft. from any septic tank or drain 23
field. Lined wetlands must be a minimum of 20 ft. 24
The designer should request a geotechnical report for the project that evaluates any 25
potential structural site instability due to extended sub-grade saturation and/or head-26
loading of the permeable layer. This includes the potential impacts to down-gradient 27
properties, especially on hills with known side-hill seeps. The report should address 28
the adequacy of the proposed stormwater treatment wetland locations and 29
recommend the necessary setbacks from any steep slopes and building foundations. 30
Landscaping (Planting Considerations) 31
When used for stormwater treatment, stormwater wetlands incorporate some of the 32
same design features as wet ponds. However, instead of gravity settling being the 33
dominant treatment process, pollutant removal mediated by aquatic vegetation (and 34
the microbiological community associated with that vegetation) becomes the 35
dominant treatment process. Thus, water volume is not the dominant design criterion 36
6 - 114 August 2010
for stormwater wetlands; rather, factors that affect plant vigor and biomass are the 1
primary concerns. It is critical to involve a Wetland Scientist, Biologist, or Landscape 2
Architect throughout the design process. 3
The wetland cells must be planted with emergent wetland plants following those of a 4
Wetland Specialist, Biologist, or Landscape Architect. 5
Soil Amendments and Protection 6
The method of construction for soil/landscape systems can affect the natural 7
selection of specific plant species. Consult a wetland specialist for site-specific soil 8
amendment recommendations. The formulation should encourage desired species 9
and discourage undesired species. Soils should be stabilized with permanent or 10
temporary cover to prevent washout due to storm flows. 11
Soil Preparation 12
The incorrect control of soil moisture is the most frequent cause of failure to establish 13
wetland plants. Inadequate water results in the desiccation of roots. Too much water 14
results in oxygen depletion in the root zone; submergence and drowning or flotation 15
of plants; and/or slow growth or plant death. 16
To maintain adequate soil moisture during plant establishment, a reliable and 17
adequate supply of water for site irrigation is needed. When feasible, a water source 18
for plant establishment is usually the stormwater treated in the wetland. However, if 19
stormwater is not available, another irrigation source must be identified to maximize 20
planting success. Adequate pumps, piping, and sprinklers or hoses must be provided 21
to allow even flow distribution. 22
The recommended sequence for maintaining soil moisture for wetland planting starts 23
with the initial saturation of soil by sprinkling or flood irrigation. For optimal plant 24
growth, the soil should be fully or partially saturated with water immediately before 25
planting and should not be allowed to completely dry out any time after planting. High 26
soil moisture must be maintained after planting for the first few weeks without 27
creating flooded conditions for more than a few hours. The best method of 28
maintaining soil saturation without excessive flooding is to start planting at the down-29
gradient end of the wetland and continue planting up-gradient, while gradually raising 30
water levels using the wetland outlet water level controls or gravity drain, if possible. 31
When planting is complete, water levels can be dropped or raised as needed to 32
maintain saturated soil conditions. Sprinklers can also be used to irrigate evenly over 33
planted areas. 34
After an entire cell is planted, the water should be maintained at a level that ensures 35
all areas of the cell continue to have saturated soil conditions between waterings. 36
This can be achieved by flood-irrigating the entire cell with enough water to allow 37
infiltration or evapotranspiration to eliminate the applied surface water within one or 38
6 - 115 August 2010
two days, or distributing water through the inlet distribution structures or down the 1
embankment side slopes and allowing this water to re-saturate the wetland soils as it 2
sheet flows across the wetland to the outlet. Weirs or outlet water control gates 3
should be removed or left open during plant establishment to prevent flooding if 4
rainfall is high or if a sprinkler or irrigator is accidentally left running. At no time 5
should flood irrigation result in complete submergence of above-ground portions of 6
installed plants. Permits may be required to use water from nearby natural aquatic 7
water bodies for temporary irrigation purposes. 8
As the wetland plants grow, they have an increased ability to transport oxygen to the 9
root zone from their leaves. Thus the plants are able to withstand longer periods of 10
flooding. However, the best technique for establishing rapid plant cover is to maintain 11
saturated soil conditions without surface flooding. The higher soil oxygen condition 12
resulting from the absence of floodwaters allows maximum root metabolism, effective 13
nutrient use, and rapid development of the plants within the wetland. This soil 14
condition should, optimally, be maintained until plants achieve complete cover (100 15
percent) or at least the minimum cover required for system startup (about 60 percent 16
to 80 percent). 17
Design and construction should allow the design water surface to be temporarily 18
modified to enable plant installation and establishment before the system is brought 19
on line. Strategies may be available depending on the project situation, schedule, 20
and site conditions. 21
If the system must go on-line the same year it is constructed, plant the 22
constructed wetland cell early in the dry season and irrigate to maintain 23
saturated soils without plant submergence or flotation, until plants are 24
sufficiently developed to operate the system at the start of the wet season. 25
If the system can remain off-line during the wet season, plant the constructed 26
wetland cell at the start of the wet season and monitor water conditions, 27
maintaining saturated soils without plant submergence or flotation by 28
irrigating or draining as necessary, until plants are sufficiently developed to 29
allow operation of the system the following year. 30
Several methods could be used to temporarily control water levels during plant 31
establishment, depending on project conditions. 32
Build the treatment wetland before the project is started, so that wetland 33
plants are established before flows are introduced 34
Keep the treatment wetland off-line until wetland plants become established 35
by bypassing the treatment wetland 36
Temporarily operate the drain of the treatment wetland as the outlet to 37
maintain water surface elevations below the design water surface level 38
6 - 116 August 2010
Plant during the dry season when water surface elevations are naturally lower 1
Pump out water to lower the wetland cell for planting and establishment 2
A wetland treatment system can typically begin operation when plant cover is at least 3
60 percent to 80 percent, which may require at least three to four months of active 4
growth. If this coverage is achieved during the first growing season after planting, the 5
wetland system can begin operating during the ensuing wet season. 6
Planting 7
Seed embankment areas above the runoff treatment design water surface and below 8
the emergency overflow water level. Areas with permanent pools that are protected 9
from erosion do not need to be seeded. 10
Consider planting trees along the west and south sides of wetlands to reduce 11
thermal heating, except that no trees or shrubs may be planted on berms meeting 12
the criteria of dams regulated for safety. Trees should be set back so that the 13
branches will not extend over the wetland. 14
Include trees and shrubs on slopes and on top of banks to increase aesthetics. If the 15
treatment wetland discharges to a phosphorus-sensitive lake or natural wetland, 16
shrubs that form a dense cover should be planted on slopes above the runoff 17
treatment design water surface on at least three sides. For banks that are berms, no 18
planting is allowed if the berm is regulated by dam safety requirements. The purpose 19
of planting is to provide shading. 20
General Maintenance Requirements 21
A drain in the wetland cell (or cells) may also be necessary to avoid surface flooding 22
during wetland plant installation and establishment. 23
Recommended Design Features 24
Where possible, the following design features should be incorporated to enhance 25
aesthetics: 26
Provide maintenance access to shallow pool areas enhanced with emergent 27
wetland vegetation. This allows the wetland to be accessible for vegetation 28
maintenance without incurring safety risks. 29
Provide side slopes that are sufficiently gentle to avoid the need for fencing 30
(3H:1V or flatter). Ponds with submerged slopes steeper than 3H:1V must be 31
fenced. 32
Provide visual enhancement with clusters of trees and shrubs. On most wetland 33
sites, it is important to amend the soil before planting because wetlands are 34
typically placed well below the native soil horizon in very poor soils. Make sure 35
that dam safety restrictions against planting do not apply. 36
6 - 117 August 2010
Consider extending the access and maintenance road along the full length of the 1
treatment wetland. The maintenance road should have a permeable surface. 2
Where right-of-way allows, orient the wetland length along the direction of 3
prevailing winds to enhance wind mixing. 4
Construction 5
Construction and maintenance considerations are the same as those for wet ponds (see 6
BMP RT.08). 7
The naturalistic grading alternative (see Figure 6-33) can be constructed by first 8
excavating the entire area to the 1.5-foot average depth. Soil subsequently excavated to 9
form deeper areas can then be deposited to raise other areas until the distribution of 10
design depths is achieved. 11
Ideally, a period of approximately one year is desirable to establish plants before the 12
system goes on-line. 13
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1 Figure 6-32: BMP RT.09 - Constructed Wetland, Option A 2
6 - 119 August 2010
1
Figure 6-33: BMP RT.09 - Constructed Wetland, Option B 2
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6.4.4.4 RT.11: COMBINED STORMWATER TREATMENT 1
WETLAND/DETENTION POND 2
General Description 3
The combined stormwater treatment wetland/detention pond is best described as a 4
wetland system that provides for the extended detention of runoff during and following 5
storm events. This BMP is useful in areas with limited right-of-way, where separate 6
runoff treatment and flow control facilities are not feasible. It is recommended that all 7
BMPs that use permanent wet pools also use facility liners. 8
Design Flow Elements 9
Flows To Be Treated 10
The sizing procedure for the combined stormwater treatment wetland and detention 11
pond is identical to that outlined for stormwater wetlands (see BMP RT.09) and for 12
combined wet/detention ponds (see BMP RT.10). Follow the procedures outlined in 13
those sections to determine the stormwater wetland size. 14
Structural Design Considerations 15
Geometry 16
The design criteria for detention ponds (see BMP FC.01) and constructed 17
stormwater treatment wetlands (see BMP RT.09) must both be met, except for the 18
following modification: 19
The minimum sediment storage depth in the first cell is 1 ft. The 6 in. of 20
sediment storage required for detention ponds and the 6 in. of sediment 21
storage in the second cell of detention ponds does not need to be added to 22
this. 23
Intent: Because emergent plants are limited to shallower water depths, the 24
deeper water created before sediments accumulate is considered detrimental 25
to robust emergent growth. Therefore, sediment storage is confined to the 26
first cell, which functions as a pre-settling cell. 27
The inlet and outlet criteria for constructed stormwater treatment wetlands (see BMP 28
RT.09) apply, with the following modifications: 29
A sump must be provided in the outlet control structure of combined facilities. 30
The detention flow restrictor and its outlet pipe must be designed according to 31
the requirements for detention ponds (see BMP FC.01). 32
Groundwater Issues 33
Live storage requirements are the same as the requirements for detention ponds (see 34
BMP FC.01). This does not apply to the dead storage component of the constructed 35
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stormwater treatment wetlands. The maximum live storage depth over the permanent 1
pool water surface should not exceed 3 ft. 2
Landscaping (Planting) 3
Check with a Wetlands Scientist; although usually, landscaping requirements for 4
constructed stormwater treatment wetlands should suffice. 5
6.5 FLOW CONTROL BMPs 6
Flow control BMPs include the following: 7
FC.01 – Detention Pond 8
General Description 9
Flow control or detention ponds are open basins that provide live storage volume to 10
enable the reduction of stormwater runoff flow rates discharged from a project site (see 11
Figure 6-34). Detention ponds are commonly used for flow control in locations where 12
space is available for an above-ground stormwater facility, but where infiltration of runoff 13
is not feasible. Detention ponds are designed to drain within 72 hours after a storm 14
event, so that the live storage volume is available for the next event. 15
Detention pond requirements discussed in this TSDM generally relate to ponds having 16
impoundment volumes less than 10 ac-ft. Larger ponds will usually require more 17
stringent geotechnical, hydraulic, and safety designs meeting federal dam guidelines. 18
Larger pond designs should be pre-approved by DPW. 19
Applications and Limitations 20
Runoff infiltration is the preferred method of flow control following appropriate runoff 21
treatment. However, in areas where infiltration is not feasible, runoff detention should be 22
implemented. 23
Detention ponds can be combined with wet pool stormwater quality treatment BMPs to 24
make more effective use of available land area (see BMP RT.10, Combined 25
Wet/Detention Pond, and BMP RT.11, Combined Stormwater Treatment 26
Wetland/Detention Pond). 27
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1 Figure 6-34: BMP FC.01 - Detention Pond 2
3
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1 Figure 6-35: Detention Pond – Cross Sections 2
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1 Figure 6-36: Outlet Control Structure, Option A 2
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1 Figure 6-37: Outlet Control Structure, Option B 2
3
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1 Figure 6-38: Alternate Overflow Structure and Trash Rack (Birdcage) 2
3
4
5
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Design Flow Elements 1
Flows To Be Detained 2
The volume and outflow design for detention ponds with a minimum 1-ft. freeboard 3
above the design water surface elevation must be determined in accordance with the 4
flow control criteria presented as part of Minimum Requirement 6 as defined in 5
Chapter 3 of this TSDM. The design water surface elevation is the highest water 6
surface elevation that is projected in order to satisfy the flow control requirements. 7
Note: If the inlet pipe is submerged below the design water surface elevation, the 8
designer should then compute the HGL of the inlet pipe to verify that backwater 9
conditions are acceptable. 10
Detention Ponds in Infiltrative Soils 11
Detention ponds may occasionally be sited on soils that are sufficiently permeable 12
for a properly-functioning infiltration system. These detention ponds have both a 13
surface discharge and a sub-surface discharge. If infiltration is accounted for in the 14
detention pond sizing calculations, the pond design process and corresponding site 15
conditions must meet all of the requirements for infiltration ponds (see BMP IN.02), 16
including a soils report, soil infiltration testing, groundwater monitoring, pre-settling, 17
and construction techniques. 18
Overflow or Bypass 19
A primary overflow (usually a riser pipe within the outlet control structure) must be 20
provided for the detention pond system to bypass the 100-year, post-developed peak 21
flow over or around the flow restrictor system. Overflow can occur when the facility is 22
full of water due to plugging of the outlet control structure or high inflows; the primary 23
overflow is intended to protect against breaching of the pond embankment (or 24
overflows of the upstream conveyance system). The design must provide controlled 25
discharge of pond overflows directly into the downstream conveyance system or 26
another acceptable discharge point. 27
A secondary inlet to the pond discharge control structure should be provided as 28
additional protection against overflows due to plugging of the primary inlet pipe to the 29
control structure. One option for the secondary inlet is a grated opening, called a 30
jailhouse window, (see Section a-a, Figure 6-35) in the control structure that 31
functions as a weir when used as a secondary inlet. The maximum circumferential 32
length of a jailhouse window weir opening must not exceed one-half the control 33
structure circumference. 34
Another common option for a secondary inlet is to allow flow to spill over the top of 35
the discharge control structure or another structure linked to the discharge control 36
structure that is fitted with a debris cage, called a birdcage (see Figure 6-38). 37
38
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Outlet Control Structure 1
Control structures are catch basins or manholes with a restrictor device for 2
controlling outflow from a facility to meet the desired performance. Riser-type 3
restrictor devices also provide some incidental oil/water separation to temporarily 4
detain oil or other floatable pollutants in runoff due to accidental spill or illegal 5
dumping. 6
The restrictor device usually consists of two or more orifices or an orifice/weir section 7
sized to meet performance requirements. 8
Discharge from the outlet pipe back to the existing or natural waterway should have 9
positive erosion protection, and if necessary, an energy dissipater structure. See 10
Section 6.6.4 for a discussion on outfalls. 11
Multiple Orifice Restrictor 12
In most cases, control structures need only two orifices -- one at the bottom and one 13
near the top of the riser, although additional orifices may optimize the detention 14
storage volume. Several orifices may be located at the same elevation if necessary 15
to meet performance requirements. 16
The minimum circular orifice diameter is 0.5 in. and the minimum vertical rectangular 17
orifice length is 0.25 in. Orifices may be constructed on a tee section. 18
In some cases, performance requirements may require the top orifice or elbow to be 19
located too high on the riser to be physically constructed (e.g., a 13-in. diameter 20
orifice cannot be positioned 6 in. from the top of the riser). In these cases, a notch 21
weir in the riser pipe may be used to meet performance requirements. 22
Consideration must be given to the backwater effect of water surface elevations in 23
the downstream conveyance system. High tailwater elevations may affect 24
performance of the restrictor system and reduce live storage volumes. 25
Riser and Weir Restrictor 26
Properly-designed weirs may be used as flow restrictors; however, they must be 27
designed to provide for primary overflow of the developed 100-year peak flow 28
discharging to the detention facility. 29
The combined orifice and riser (or weir) overflow may be used to meet performance 30
requirements; however, the design must still provide for primary overflow of the 31
developed 100-year peak flow, assuming all orifices are plugged. 32
Emergency Overflow Spillway 33
In addition to the overflow provisions described above, detention ponds must have 34
an emergency overflow spillway that is sized to pass the 100-year, post-developed, 35
un-detained peak flow in the event of total control structure failure, such as blockage 36
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of the control structure outlet pipe, or extreme inflows. Emergency overflow spillways 1
are intended to control the location where flows overtop the pond perimeter and 2
direct overflows into the downstream conveyance system or other acceptable 3
discharge point. The bottom of the emergency overflow spillway must be set at the 4
design frequency water surface elevation. 5
As an option, emergency overflow may be provided by a manhole fitted with a 6
birdcage, as shown in Figure 6-38. Where an emergency overflow spillway would 7
discharge to a steep slope, consideration should be given to providing an emergency 8
overflow structure in addition to the spillway. 9
The emergency overflow spillway must be armored with riprap that is sized in 10
conformance with the guidelines in Chapter 4 of FHWA publication HEC-11. The 11
spillway must be armored across its full width and also must be armored down the 12
embankment, as shown in Figure 6-35. 13
Emergency overflow spillway designs as shown in Figure 6-35 must be analyzed as 14
broad-crested trapezoidal weirs. The minimum bottom width of the weir should be 15
6 ft. 16
Emergency overflow spillway designs using a manhole fitted with a birdcage, as 17
shown in Figure 6-38, should be analyzed as a sharp-crested weir with no end 18
contractions to pass the 100-year, post-developed, un-detained peak flow. 19
Structural Design Considerations 20
Geometry 21
Pond inflows must enter through a conveyance system separate from the outlet 22
control structure and outflow conveyance system. Maximizing the distance between 23
the inlet and outlet is encouraged to promote sediment trapping. 24
Pond bottoms must be level and a minimum of 0.5 ft. below the outlet invert elevation 25
to provide sediment storage. 26
Berms, Baffles, and Slopes 27
Interior side slopes up to the emergency overflow water surface should not be 28
steeper than 3H:1V, unless a fence is provided (see ―Fencing‖, below). 29
Exterior side slopes must not be steeper than 2H:1V, unless they are analyzed for 30
stability by a Geotechnical Engineer. 31
Pond walls may be vertical retaining walls subject to the following: 32
Cast-in-place wall sections must be designed as retaining walls. A Licensed 33
Civil Engineer with structural expertise must stamp structural designs for 34
cast-in-place walls. All construction joints must be provided with water stops. 35
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Gravity type block walls (rockery or ecology block types) may be used to no 1
more than 9 ft. vertical height. These gravity walls must be backfilled with a 2
free-draining, structural backfill using either well-graded aggregate or a 3
geotextile to act as a filter layer to protect soil fines from migrating through 4
the block openings. 5
Walls must be placed on stable, well-consolidated native material with 6
suitable bedding. Walls must not be placed in fill slopes unless the slopes 7
have been analyzed in a geotechnical report for stability and constructability. 8
A fence is provided along the top of the wall. 9
Although the entire pond perimeter may be retaining walls, it is recommended 10
that at least 25 percent of the pond perimeter be a vegetated soil slope not 11
steeper than 3H:1V. 12
If the entire pond perimeter is to be retaining walls, ladders should be 13
provided on the full height of the walls for safe access by maintenance staff. 14
Embankments 15
The minimum top width of berm embankments should be 5 ft. or as recommended by 16
a Geotechnical Engineer. If used for maintenance access, the berm tops should be a 17
minimum of 12 ft. wide. 18
Pond berm embankments are usually constructed on native consolidated soil (or 19
adequately compacted and stable fill soils analyzed by a Geotechnical Engineer), 20
and free of loose surface soil materials, roots, and other organic debris. 21
Pond berm embankments greater than 4 ft. high must be constructed by excavating 22
a key trench equal to 50 percent of the berm embankment‘s cross-sectional height 23
and width, unless otherwise specified by a Geotechnical Engineer. 24
Anti-seepage filter-drain diaphragms (pipe collars) must be placed on outflow pipes 25
in berm embankments impounding water with depths greater than 8 ft. at the design 26
water surface. The size of the pipe collars should be such to extend the seepage 27
flow path at least 1.5 times the length of the buried pipe section. 28
Groundwater Issues 29
Identification and Avoidance 30
Flow control BMPs must be constructed above the seasonal high groundwater table. 31
Storage capacity and proper flow attenuation are compromised if groundwater levels 32
exceed the live storage elevations. The project should locate the pond so that there 33
is a separation between the local groundwater table elevation and the bottom of the 34
proposed BMP. In some cases, this may require that a much shallower pond be 35
constructed in order to function properly. 36
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The groundwater table elevation in and around the flow control facility needs to be 1
determined early in the project. This can be done by installing piezometers at the 2
BMP location and taking water table readings over at least one wet season. 3
Designers should look for opportunities to provide flow control to an equivalent area 4
in the project that discharges to the same watershed. 5
Site Design Elements 6
Setback Requirements 7
Detention ponds must be a minimum of 5 ft. from any property line or vegetative 8
buffer. This distance may need to be increased based on the permit requirements of 9
the local jurisdiction. 10
Detention ponds must be 100 ft. from any septic tank or drain field. 11
The designer should request a geotechnical report for the project that evaluates any 12
potential structural site instability due to extended sub-grade saturation or head 13
loading of the permeable layer, including the potential impacts to down-gradient 14
properties, especially on hills with known side-hill seeps. The report should address 15
the adequacy of the proposed detention pond locations and recommend the 16
minimum excavation and fill slopes, plus necessary setbacks from any steep slopes 17
and building foundations. 18
Landscaping (Planting Considerations) 19
Earthen ponds should be vegetated with a good erosion control grass seed mix that 20
is tolerant of being submerged for short periods of time. All disturbed soil surfaces 21
should be covered with topsoil or mulch amended and seeded with an erosion 22
control grass mix as recommended by NRCS or DAWR for the site and climate 23
conditions. 24
Fencing 25
Ponds having submerged slopes steeper than 3H:1V must be fenced to restrict 26
public access. 27
6.6 STORMWATER FACILITY COMPONENTS 28
6.6.1 PRE-SETTLING/SEDIMENTATION BASIN 29
General Description 30
A pre-settling basin provides pre-treatment of runoff to remove suspended solids that 31
can impact other primary runoff treatment BMPs (see Figures 6-39 and 6-40). 32
33
34
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Applications and Limitations 1
The most attractive aspect of a pre-settling basin is its isolation from the rest of the 2
facility. Pre-settling basins allow sediment to fall out of suspension. However, they do not 3
detain water long enough for the removal of most pollutants (such as some metals). Pre-4
settling basins are frequently used as pre-treatment for downstream infiltration facilities 5
and to protect more sensitive facilities, such as constructed stormwater treatment 6
wetlands, from excessive sediment loads. Runoff treated by a pre-settling basin may not 7
be discharged directly to a receiving water body; it must be further treated by a 8
stormwater quality treatment BMP. 9
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1 Figure 6-39: Typical Pre-settling/Sedimentation Basin 2
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1 Figure 6-40: Pre-settling/Sedimentation Basin – Alternate Sections 2
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Design Flow Elements 1
Flows To Be Treated 2
A pre-settling basin should be designed as a wet pool. The wet pool volume must be 3
at least 30 percent of the total volume of runoff from the three-year frequency storm 4
event. The wet pool volume should be allowed to dry out between rain storms, either 5
by infiltration and evaporation, or by providing a pipe outlet with a perforated type 6
riser to allow for low-flow percolation flows. 7
Overflow or Bypass 8
Pre-settling basin design must take into consideration the possibility of overflows. A 9
designed overflow section should be constructed along the pre-settling basin 10
embankment to allow flows to exit at a non-erosive velocity during the design 11
frequency storm event. The overflow may be set at the permanent pool level. The 12
use of an aquatic bench with emergent vegetation around the perimeter helps with 13
water quality. 14
Any basin design must consider both a primary discharge and an emergency 15
overflow discharge in accordance with the requirements of BMP FC.01. Overflow 16
non-piped outlets can also function for the emergency case as long as they are sized 17
for the maximum peak inlet conveyance flow (or the 100-year frequency storm with 18
1 ft. of freeboard). 19
Inlet Structure 20
The runoff treatment volume should be discharged uniformly and at a low velocity 21
into the pre-settling basin to maintain near-quiescent conditions, which are 22
necessary for effective treatment. It is desirable for the heavier-suspended material 23
to drop out near the front of the basin. Energy-dissipation devices may be necessary 24
to reduce inlet velocities that exceed 3 ft. per second. 25
Outlet Control Structure 26
The outlet structure conveys the runoff treatment volume from the pre-settling basin 27
to the primary treatment BMP (e.g., an infiltration pond). The outlet control structure 28
should skim the surface water. It can be a piped outlet structural overflow weir (see 29
Figure 6-38), or it can be created as an earthen berm, gabion, concrete, or riprap 30
wall along the separation embankment preceding the primary treatment BMP. 31
Structural Design Considerations 32
Geometry 33
A long, narrow basin is preferred because it is less prone to short-circuiting and 34
tends to maximize the available treatment area. The length-to-width ratio should be 35
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at least 3:1, and preferably 5:1. The inlet and outlet should be at opposite ends of the 1
basin. 2
Berms, Embankments, and Slopes 3
Berm embankments must be constructed on suitably-prepared native soil (or 4
adequately-compacted and stable fill soils analyzed by a geotechnical report) and 5
free of loose surface soil materials, roots, and other organic debris. 6
Exposed earth on the side slopes and bottom should be sodded or seeded with the 7
appropriate seed mixture as soon as practicable. If necessary, geotextile or matting 8
may be used to stabilize slopes until seeding or sodding become established. 9
If composed of a structural retaining wall, interior side slopes may be nearly vertical, 10
as long as maintenance access is provided. Otherwise, they should be no steeper 11
than 3H:1V. Exterior embankment slopes should be 2H:1V or less. The bottom of the 12
basin should have a 2 percent slope to allow for complete drainage. The minimum 13
depth must be 4 ft. and the maximum depth must be 6 ft. 14
Liners 15
Infiltration from the typical sediment basin is desirable, except where the bottom of 16
the basin is less than 3 ft. above the limestone bedrock in the aquifer recharge areas 17
of northern Guam. In that case, the pre-treatment pond must be lined with an 18
impermeable membrane. 19
Site Design Elements 20
Setback Requirements 21
Pre-settling basins must be a minimum of 5 ft. from any property line or vegetative 22
buffer. This distance may need to be increased based on the permit requirements of 23
the local jurisdiction. 24
Pre-settling basins must be 100 ft. from any septic tank or drain field, except wet 25
vaults, which must be a minimum of 20 ft. If lined with an impermeable membrane, 26
the pre-settling basins may be within 20 ft. of a drain field. 27
The designer should request a geotechnical report for the project that evaluates any 28
potential structural site instability due to extended sub-grade saturation or head-29
loading of the permeable layer, including the potential impacts to down-gradient 30
properties (especially on hills with known side-hill seeps). The report should address 31
the adequacy of the proposed pre-settling basin locations and recommend the 32
necessary setbacks from any steep slopes and building foundations. 33
Safety, Signage, and Fencing 34
Basins that are readily-accessible to populated areas should incorporate all possible 35
safety precautions. Dangerous outlet facilities should be protected by an enclosure. 36
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Warning signs should be used wherever appropriate and placed so that at least one 1
is clearly visible and legible from all adjacent streets, sidewalks, or paths. 2
Maintenance 3
The failure of large impoundment structures can cause significant property damage 4
and even loss of life. Impoundment structures should be regularly inspected for signs 5
of failure, such as seepage or cracks in the walls or berm. 6
Pre-settling basins are less likely than wet ponds to build up excessive levels of 7
heavy metals from sediments washed off of impervious areas. Routine maintenance 8
should remove and properly dispose of any significant sediment deposits. Sediment 9
should be removed every three to five years or when 6 in. to 12 in. have 10
accumulated, whichever comes first. More frequent removal of sediment from the 11
pre-settling basin may be less costly over the same time period than a one-time 12
cleaning of the entire basin. 13
6.6.2 FLOW SPLITTERS 14
Although volume-based (wet pool) runoff treatment BMPs must be designed as on-line 15
facilities, many flow rate-based runoff treatment BMPs can be designed as either on-line 16
or off-line. On-line systems allow flows above the runoff treatment design flow to pass 17
through the facility at a lower pollutant-removal efficiency. However, it is sometimes 18
desirable to restrict flows to an off-line runoff treatment facility and bypass the remaining 19
higher flows around the BMP. This can be accomplished by splitting flows in excess of 20
the runoff treatment design flow upstream of the facility, and diverting higher flows to a 21
bypass pipe or channel. The bypass typically enters a detention pond or the downstream 22
receiving drainage system, depending on flow control requirements. In most cases, it is 23
the designer‘s choice whether runoff treatment facilities are designed as on- or off-line. 24
A crucial factor in designing flow splitters is ensuring that low flows are delivered to the 25
treatment facility up to the runoff treatment design flow rate. Above this rate, additional 26
flows are diverted to the bypass system, with minimal increase in head at the flow splitter 27
structure, to avoid surcharging the runoff treatment facility under high flow conditions. 28
Flow splitters are typically manholes or vaults with concrete baffles. In place of baffles, 29
the splitter mechanism may be a half-tee section with a solid top and an orifice in the 30
bottom of the tee section. A full tee option may also be used, as described below in 31
―General Design Criteria.‖ One possible design for flow splitters is shown in Figure 6-41. 32
Other equivalent designs that achieve the result of splitting low flows and diverting 33
higher flows around the facility are also acceptable. 34
General Design Criteria 35
A flow splitter must be designed to deliver the stormwater quality treatment design flow 36
rate to the runoff treatment facility. For the wet pond and created wetland, which are 37
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sized based on volume, use the stormwater quality treatment design flow rate to design 1
the splitter. 2
The typical flow splitter is based on a weir design. The top of the weir must be located at 3
the water surface for the design flow. Excess spillover flows enter the bypass line. 4
The maximum head must be minimized for flow in excess of the runoff treatment design 5
flow. Specifically, flow to the runoff treatment facility in the 100-year event must not 6
increase the runoff treatment design flow by more than 10 percent. 7
Special applications may require the use of a modified flow splitter. The baffle wall may 8
be fitted with a notch and adjustable weir plate to proportion runoff volumes other than 9
high flows. 10
Ladder or step-and-handhold access must be provided. If the weir wall is higher than 11
36 in., two ladders, one on either side of the wall, must be used. 12
Materials 13
The splitter baffle may be installed in a manhole or vault. 14
The baffle wall must be made of reinforced concrete or another suitable material 15
resistant to corrosion and have a minimum 4-in. thickness. The minimum clearance 16
between the top of the baffle wall and the bottom of the manhole cover must be 4 ft.; 17
otherwise, dual access points should be provided. 18
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1 Figure 6-41: Flow Splitter 2
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All metal parts must be corrosion-resistant. Examples of preferred materials include 1
aluminum, stainless steel, and plastic. Avoid the use of zinc and galvanized materials 2
because of their aquatic toxicity potential, when substitutes are available. Painting metal 3
parts for corrosion resistance is not allowed because paint does not provide long-term 4
protection. 5
6.6.3 FLOW-SPREADING OPTIONS 6
Flow spreaders function to uniformly spread flows across the inflow portion of runoff 7
treatment facilities (such as a biofiltration swale, or a vegetated filter strip). Seven flow 8
spreader options are presented in this section: 9
Option A – Anchored plate 10
Option B – Concrete sump box 11
Option C – Notched curb spreader 12
Option D – Through-curb ports 13
Option E – Interrupted curb 14
Option F – Flow Spreader Trench 15
Option G – Flow Spreader Swale 16
Options A through C, F, and G can be used for spreading flows that are concentrated. 17
Any one of these options can be used when spreading is required by the facility design 18
criteria. Options A through C, F, and G can also be used for un-concentrated flows; in 19
some cases they must be used, such as to correct for moderate grade changes along a 20
vegetated filter strip. 21
Options D and E are only for runoff that is already in a sheet flow condition and enters a 22
vegetated filter strip, continuous inflow biofiltration swale, or infiltration trench. 23
General Design Criteria 24
Flows entering the spreader structure should be sub-critical non-erosive velocities. If 25
required, energy dissipation should be provided by using rock or concrete pads or stilling 26
structures (typically an inlet structure with flow overflowing the grated top into the 27
spreader). 28
Option A – Anchored Plate 29
Anchor plates can be used wherever concentrated flows need to be spread for a 30
uniform sheet flow condition such as at biofiltration swales and engineered 31
dispersion BMPs. An anchored plate flow spreader (see Figure 6-42) must be 32
preceded by a sump having a minimum depth of 8 in. and a minimum width of 24 in. 33
If not otherwise stabilized, the sump area must be rock riprap protected to reduce 34
erosion and dissipate energy. 35
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The top surface of the flow spreader plate must be level, projecting a minimum of 1
2 in. above the ground surface of the runoff treatment facility, or V-notched with 2
notches 6 in. to 10 in. on center, and 1 in. to 6 in. deep (use shallower notches with 3
closer spacing). Alternative designs may also be used. 4
A flow spreader plate must extend horizontally beyond the bottom width of the facility 5
to prevent water from eroding the side slope. The horizontal extent should protect the 6
bank for all flows up to the 100-year flow or the maximum flow that enters the runoff 7
treatment facility. 8
Flow spreader plates must be securely fixed in place, and may be made of wood, 9
metal, fiberglass-reinforced plastic, or other durable material. If wood, pressure-10
treated 4-in. by 10-in. lumber/landscape timbers are acceptable. 11
Anchor posts must be 4-in. square concrete, tubular stainless steel, or other material 12
resistant to decay. 13
Option B – Concrete Sump Box 14
The concrete sump box is used mainly as a spreader for biofiltration swale BMPs. 15
The wall of the downstream side of a rectangular concrete sump box (see Figure 6-16
20) must extend a minimum of 2 in. above the treatment bed. This serves as a weir 17
to spread the flows uniformly across the bed. 18
The downstream wall of a sump box must have wing walls at both ends. Sidewalls 19
and returns must be slightly higher than the weir so that erosion of the side slope is 20
minimized. 21
Concrete for a sump box can be either cast-in-place or pre-cast, Both types must be 22
reinforced with rebar or wire mesh per standard structural design requirements. 23
Sump boxes must be placed over bases consisting of 4 in. of crushed rock, 5/8-in. 24
minus, to help ensure the sump remains level. 25
Option C – Notched Curb Spreader 26
Notched curb spreader sections (see Figure 6-43) are usually made of extruded or 27
pre-cast concrete laid side-by-side and level. Typically, five teeth per 4-ft. section 28
provides good spacing. The space between adjacent teeth forms a V-notch. 29
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1 Figure 6-42: Flow Spreader – Anchor Plate 2
3
4
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1 Figure 6-43: Flow Spreader – Notched Curb Spreader 2
Option D – Through-Curb Ports 3
Un-concentrated flows from paved areas entering vegetated filter strips or 4
continuous inflow biofiltration swales can use curb ports (see Figure 6-44) or 5
interrupted curbs (Option E) to allow flows to enter the strip or swale. Curb ports use 6
fabricated openings that allow concrete curbing to be poured, pre-cast or extruded, 7
with an opening through the base to admit water to the runoff treatment facility. 8
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Openings in the curb must be at regular intervals — at least every 6 ft. (minimum). 1
The width of each curb port opening must be a minimum of 11 in. Approximately 2
15 percent or more of the curb section length should be in open ports, and no port 3
should discharge more than approximately 10 percent of the flow. 4
Option E – Interrupted Curb 5
Interrupted curbs are sections of curb placed to have gaps spaced at regular 6
intervals along the total width (or length, depending on the facility) of the treatment 7
area. At a minimum, gaps must be every 6 ft. to allow for the distribution of flows into 8
the treatment facility before the flows become too concentrated. The opening must 9
be a minimum of 11 in. As a general rule, no opening should discharge more than 10
10 percent of the overall flow entering the facility. 11
12
13 Figure 6-44: Flow Spreader – Through-Curb Port 14
Option F – Flow Spreader Trench 15
Flow spreader trenches are the preferred outlet type device for discharge of 16
concentrated flows from stormwater quality treatment and detention facilities. They 17
act as both infiltration trenches, and they allow sheet flow discharge over the top for 18
flows in excess of the infiltration capacity. They provide a more natural type of 19
discharge regime where placed parallel to streams and wetlands, a better alternative 20
than direct pipe outfalls to water bodies. 21
Flow spreader trenches are also the usual flow spreader device where concentrated 22
flows must be converted to sheet flow for distribution to engineered wetlands and 23
6 - 145 August 2010
vegetated filter strips. Two types of flow spreader trenches are shown in Figures 6-1
45 and 6-46. 2
3
4 Figure 6-45: Flow Spreader Trench Type 1 5
6
7
8
6 - 146 August 2010
1 Figure 6-46: Flow Spreader Trench Type 2 2
The Type 1 trench works well for energy dissipation of higher velocity incoming flow. 3
The pipe sizes and trench length are sized to accommodate the design flow rate 4
while limiting the flow depth over the weir to 3 in. or 4 in. maximum. The maximum 5
hydraulic capacity of these flow dispersal trenches are limited by the practical flow 6
parameters, so that there is no possibility of dislodging the trench backfill material, 7
and pipe flow velocities in the perforated pipe are no more than 5 fps to 6 fps. 8
Option G – Flow Spreader Swale 9
A flow spreader swale is simply a ditch with a level berm along the downhill side. The 10
ditch should be sized for the design flow with a flow velocity limited to 2 ft. or 3 ft. per 11
second with the outlet being the overtopping of the downhill level berm. The swale 12
should be trapezoidal in shape with side slopes no steeper than 3H:1V for ease in 13
maintenance mowing. The level berm should be at least 4 ft. in top width constructed 14
of compacted soil and grassed with a heavy sod-forming type plant mix. 15
The berm overflow depth should not exceed 1 in. to 2 in. to protect against flow 16
cutting and rutting. These level flow spreaders are most applicable for use with 17
engineered dispersion BMPs and vegetated filter strips. The level berm should be 18
protected with an erosion control type geotextile if placed into use prior to full 19
development of the grass sod. 20
6 - 147 August 2010
Level flow spreaders should have frequent maintenance checking for any signs of 1
erosion or soil rutting. Any damaged areas on the level berm should be immediately 2
repaired and reseeded. The level flow swales also require frequent mowing, and 3
should not be used in areas inaccessible to this requirement. 4
6.6.4 OUTFALLS 5
Properly-designed outfalls are critical to reducing the chance of adverse impacts as the 6
result of concentrated discharges from pipe systems and culverts, both on-site and 7
downstream. 8
Outfall systems include rock splash pads, flow spreader trenches, gabion, or other 9
energy pipe used to convey flows down a steep or sensitive slope with appropriate 10
energy dissipation at the discharge end. 11
General Design Criteria 12
At a minimum, all outfalls must be provided with a rock splash pad (see Table 6-9 and 13
Figure 6-47). 14
Where discharging from a stormwater quality treatment BMP to a natural stream or 15
wetland, the preferred outfall is the flow spreader trench (see Figures 6-45 and 6-46). 16
The flow spreader trench provides a combination infiltration/sheet flow discharge that is 17
more natural and protective of the existing water body. 18
For outfalls with a design velocity greater than 8 ft. to 10 ft per second, an engineered 19
energy dissipater may be required. There are many possible designs. One example for 20
typical-sized storm drainage pipe outfalls with higher velocities is the gabion basket 21
protection (see Figure 6-48). Note: The gabion outfall detail shown in Figure 6-48 is 22
illustrative only. A design engineered to specific site conditions must be developed. 23
Tight-line systems may be needed to prevent aggravation or creation of a downstream 24
erosion problem. 25
Outfalls to marine waters require specific engineering designs to account for the 26
additional corrosion effects and energy of the receiving waters, and to minimize impacts 27
to the ecology of the tidal zone. In marine waters, rock splash pads and gabion 28
structures are not recommended. Rock splash pads can be destroyed by wave action, 29
and gabion baskets will corrode in saltwater and, potentially, be dislocated by wave 30
action. 31
If the discharge cannot be made above the mean high water in marine waters, tight line 32
systems are used. The energy dissipation is performed within the submerged section of 33
the outfall pipeline/structures. The outfall pipeline is normally trenched to discharge 34
below extreme low water, often times using a ‗T‘ diffuser on the end. The outfall must be 35
protected by heavy riprap blankets from wave and tidal current action, and boat 36
anchorage. 37
6 - 148 August 2010
Where discharge flows exceed 10 ft. to 12 ft. per second with a Froud number above 1, 1
engineered energy dissipaters should be used. They should be designed in accordance 2
with FHWA publication HEC-14 guidelines. There are also public domain computer 3
programs to assist with the selection and design of dissipater structures such as the 4
FHWA‘s HY-8, the USDA‘s WinFlume32, and the Nebraska Department of Roads‘ 5
BCAP (Broken-Back Culvert Analysis Program). 6
7
Discharge Velocity at Design Flow (ft./s)
Required Protection Minimum Dimensions
Type Thickness Width Length Height
0-5 Rock lining1 1 ft. Diameter + 6 ft.
8 ft. or 4 x diameter, whichever is greater
Crown + 1 ft.
5-10 Riprap2 2 ft.
Diameter + 6 ft. or 3 x diameter, whichever is greater
12 ft. or 4 x diameter, whichever is greater
Crown + 1 ft.
Table 6-9: Rock Protection at Outfalls 8
1 Rock lining is quarry spalls (pit run) with gradation as follows: 9 Passing 8-in.-square sieve: 100% 10 Passing 3-in.-square sieve: 40% to 60% maximum 11 Passing 3/4-in.-square sieve: 0% to 10% maximum 12
2 Riprap to be reasonably well-graded with gradation as follows: 13 Maximum stone size: 24 in. (nominal diameter) 14 Median stone size: 16 in. 15 Minimum stone size: 4 in. 16
Note: Riprap sizing on an outlet channel is assumed to be governed by side slopes of 17 approximately 3H:1V. 18 19
6 - 149 August 2010
1 Figure 6-47: Pipe/Culvert Outlet Rock Protection 2
3
4
5
6 - 150 August 2010
1
Figure 6-48: Gabion Outlet Protection 2
6 - 151 August 2010
Tight-Line Systems 1
Mechanisms that reduce runoff velocity prior to discharge from an outfall are 2
encouraged. Two of these mechanisms are drop manholes and the rapid expansion 3
of pipe diameter. 4
Bank stabilization, bioengineering, and habitat features may be required for disturbed 5
areas. Outfall structures should be located and configured to minimize impacts to the 6
riparian habitat, and in particular, avoid impacts below the Ordinary High Water of 7
natural water bodies. 8
6.6.5 SOIL AMENDMENTS 9
General Description 10
Soil amendments, including compost and other organic materials, help restore 11
the health of the soil and increase environmental functions such as rainwater 12
infiltration and natural detention, evapotranspiration, and plant health. Soil 13
amendments can help prevent or minimize adverse stormwater impacts during 14
construction and are used along with vegetation as a permanent runoff treatment 15
BMP. Compost is a versatile material that can be used as a component in many 16
other permanent and temporary stormwater BMPs. 17
Compost-amended soils can be modeled as pasture on native soil. The final 18
organic content of these soils should be 10 percent for all areas, excluding turf 19
areas, which are expected to receive a high amount of foot traffic. Turf (lawn) 20
areas with high foot traffic must have a 5 percent final organic content. 21
Applications and Limitations 22
Soil amendments can be used in most unpaved areas within the project. If soil 23
amendments are applied as a blanket, they perform erosion control functions 24
immediately by providing a cover to bare soils. When incorporated into the soil, 25
they increase infiltration and adsorption of metals and aid in the uptake of 26
nutrients. They also enhance vegetation growth and plant establishment. 27
Compost provides an excellent growing medium for roadside vegetation. 28
Traditional highway construction methods typically result in the excavation and 29
removal of the area‘s topsoil. Roadway embankments are then constructed from 30
material that has few nutrients, is low in organic material, and is compacted to 95 31
percent maximum density. Adding compost to roadway slopes and ditches 32
provides soil cover, improves soil fertility and texture, and greatly improves the 33
vegetative growth and soil stability, thereby reducing erosion. 34
Organic soil amendments soak up water like a sponge and store it until it can be 35
slowly infiltrated into the ground or taken up by plants. In some BMP applications, 36
6 - 152 August 2010
the volume of compost can be sized to absorb and hold the runoff treatment 1
storm. 2
Compost is an excellent filtration medium, which provides treatment for highway 3
runoff. Compost has a high cation exchange capacity (CEC) that chemically traps 4
dissolved heavy metals and binds them to the compost material. Oils, grease, 5
and floatables are also removed from stormwater as it is filtered through the 6
compost. 7
Compost is very absorbent when dry, but when saturated it has a high infiltration 8
rate. Therefore, greater storm events can pass through compost medium without 9
hindering the infiltration rates of underlying soils or drain materials. Compost has 10
also been shown to improve the infiltration rates of underlying soils, even till soils. 11
Placement of a compost blanket on bare soil helps stabilize the soil and prevent 12
surface erosion by intercepting rainfall. This type of application changes the 13
texture and workability of the soil, lengthens the acceptable seeding windows, 14
and encourages plant growth. 15
Compost soil amendments can be used in the construction phase of projects as 16
compost berms and compost socks in lieu of conventional geotextile silt fences 17
for sediment control. While being an effective sediment trap during the 18
construction phase, compost berms are advantageous in that they can be bladed 19
out at the construction site, which avoids bid items for the haul and disposal of 20
silt fences. If the permanent stormwater design involves use of compost-21
amended vegetated filter strips, a batch of compost can be used as sediment 22
control in a berm, then the berm can be bladed out along a highway roadside, 23
where it can be used as part of vegetated filter strip construction. Compost socks 24
can be left in place and will deteriorate with time. 25
Maintenance 26
Compost, as with sand filters or other filter mediums, can become plugged with 27
fines and sediment, which may require removal and replacement. Including 28
vegetation with compost helps prevent the medium from becoming plugged with 29
sediment by breaking up the sediment and creating root pathways for stormwater 30
to penetrate into the compost. It is expected that soil amendments will have a 31
removal and replacement cycle; however, this time frame has not yet been 32
established. 33
34
35
6 - 153 August 2010
6.7 OPERATION AND MAINTENANCE 1
Inadequate maintenance is a common cause of failure for stormwater control facilities. 2
Proper maintenance requires regular inspection and prompt repair and cleaning of 3
structures when needed. The designer should simplify this process by providing designs 4
that promote easy access for maintenance inspections and regular maintenance work. 5
The stormwater designer should follow these guidelines as a minimum: 6
Review other stormwater BMP installations and observe those items that are 7
ineffective, poorly-maintained, and/or working correctly for what should not be or 8
what should be incorporated into the planned BMPs. 9
Review all preliminary BMP designs with DPW‘s maintenance staff for their input 10
on the maintainability. 11
Provide safe, convenient access to the BMPs for ease of regular maintenance. 12
This should include properly-designed traffic pull-out areas that allow for safe 13
egress in and out of traffic, with surfaced parking and walking access. Working 14
pads and access ramps should be suitable for the facility in regards to the 15
equipment needed to maintain the facility, such as surfaced pads large enough to 16
maneuver trucks for cleaning and loading, ramps surfaced on slopes suitable for 17
the equipment, parking in front of gates for inspection vehicles, etc. 18
Provide vehicle access for cleaning of all inlet and outlet control structures. 19
Provide pullout locations for maintenance vehicles with trailers where mowing is 20
required. 21
Underground structures should have suitable work pads around the access 22
opening for equipment, with access openings suitable for both inspection 23
manhole access and larger sediment/trash removal equipment. 24
Provide lighting and ventilation where regular access must be made in enclosed 25
space locations such as underground sumps and pump chambers. 26
For critical drainage and flood control facilities, provide emergency failure alarms 27
based on high water levels or pump failure, with red flashing light and signs at 28
the site plus a wireless signal to DPW‘s maintenance headquarters. 29
30
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7 - 1 August 2010
7 CONSTRUCTION SITE BEST MANAGEMENT PRACTICES 1
This chapter of the Transportation Stormwater Drainage Manual (TSDM) provides designers 2 with specific guidelines and criteria on the proper selection, design, and application of 3 construction site stormwater management techniques. A selection process is presented, 4 along with design considerations for each Best Management Practice (BMP). 5
Construction site BMPs are applied during construction activities to reduce the pollutants in 6 stormwater discharges throughout construction. These construction site BMPs provide both 7 temporary erosion and sediment control, as well as control for potential pollutants other than 8 sediment. There are five categories of BMPs suitable for controlling potential pollutants on 9 construction sites. They are: 10
Soil stabilization practices 11
Sediment control practices 12
Wind erosion control 13
Tracking control practices 14
Non-stormwater controls 15
It is generally accepted that practices that perform well by themselves can be 16 complemented by other BMPs to raise the collective effectiveness level for erosion control 17 and sediment retention. 18
Effective erosion and sediment control planning relies on a system of BMPs such as 19 mulches for source control, fiber rolls on slopes for reducing runoff velocities, and silt fences 20 at the toe of slopes for capturing sediment, among others. 21
To meet regulatory requirements and protect the site resources, every project must include 22 an effective combination of erosion and sediment control measures. These measures must 23 be selected from all of the BMP categories presented in this section -- soil stabilization 24 practices, sediment control practices, tracking control practices, and wind erosion control 25 practices. 26
Additionally, the project plan must include non-stormwater controls, waste management, 27 and material pollution controls. 28
Table 7-1 is a matrix of the construction site BMPs that are generally used during 29 construction. Detailed descriptions and guidance regarding the implementation of these 30 BMPs may be found in later sections of this chapter. The individual BMPs, designated by an 31 “X” in Table 7-1 as being applicable to a particular, typical construction activity, is a guideline 32 only, and the final selection of BMPs will depend on the actual site conditions and 33 contractor’s work elements, staging, schedule, and site management philosophy. 34
35
7 - 2 August 2010
The last section of this chapter discusses post construction source control BMPs, 1 sometimes also known as “maintenance BMPs”. The objective of implementing maintenance 2 BMPs is to provide preventative measures to ensure that maintenance activities are 3 conducted in a manner that reduces the amount of pollutants discharged to surface waters 4 through DPW’s stormwater drainage systems. DPW’s maintenance activities involve the use 5 of a variety of products. Under normal, intended conditions of use, these materials are not 6 considered “pollutants of concern.” However, if these products are used, stored, spilled, or 7 disposed of in a way that may cause them to contact stormwater or enter stormwater 8 drainage systems, they may become a concern for water quality. 9
10
7 - 3 August 2010
Dem
olis
h Pa
vem
ent a
nd
Stru
ctur
es
Cle
ar a
nd G
rub
Con
stru
ct A
cces
s R
oads
Gra
ding
(inc
. cut
an
d fil
l slo
pes)
Cha
nnel
Ex
cava
tion
Tren
chin
g an
d U
nder
grou
nd
Dra
inag
e
Util
ity T
renc
hing
Util
ity In
stal
latio
n
Subg
rade
pr
epar
atio
n
Base
Pav
ing
Pavi
ng
Saw
Cut
ting
Join
t Sea
ling
Grin
d/G
roov
e
Stru
ctur
e Ex
cava
tion
Erec
t Fal
sew
ork
Brid
ge/S
truct
ure
Con
stru
ctio
n
Rem
ove
Fals
ewor
k
Strip
ing
Mis
c. C
oncr
ete
Wor
k
Soun
d an
d R
etai
ning
Wal
ls
Plan
ting
and
Irrig
atio
n
Con
tract
or
Activ
ities
Trea
tmen
t BM
P C
onst
ruct
ion
Construction Best Management Practices
Soil Stabilization Practices Scheduling X X X X X X X X X X X X X X X X X X X X X X Preservation of Existing Vegetation X X X X X X X X X Hydraulic Mulch X X X X X X X
Hydroseeding X X X X X X X Soil Binders X X X X X X X X Fiber Mulch X X X X X X X X X X X X
Geotextile, Plastic Covers, and Erosion Control Blankets/Mats
X X X X X X X X X X X X
Temporary Concentrated Flow Conveyance Controls X X X X X
Streambank Stabilization X X X X X X X Sediment Control Practices Silt Fence X X X X X X X X X X X X X X Sediment Basin X X X X X X Sediment Traps X X X X X X X X X X X X X X Check Dams X X X X X X X X X X X X X X Fiber Rolls/Wattles X X X X X X X X X X X X X X Gravel Bag Berm X X X X X X X X X X X X X X
Street Sweeping and Vacuuming X X X X X X X X X X X X X X Storm Drain Inlet Protection X X X X X X X X X X X X X X Streambank Sediment Control X X X X X X X X X X X X X X
Wind Erosion Control Tracking Control Practices
Stabilized Construction Entrance/Exit
X X X X
Stabilized Construction Roadway X X X X Entrance/Outlet Tire Wash X X X X X
Non-stormwater Controls Water Conservation Practices X X X X X X X X X X X X X X X X X
De-Watering Operation X X X X X X X X X X X X Temporary Stream Crossing X X X X X X X X Clear Water Diversion X X X X X X X X Potable Water/Irrigation X X X Vehicle and Equipment Cleaning X X X X X X X X X X X X X X X X X X X X X X X X Vehicle and Equipment Fueling X X X X X X X X X X X X X X X X X X X X X X X X Vehicle and Equipment Maintenance X X X X X X X X X X X X X X X X X X X X X X X X
7 - 4 August 2010
Dem
olis
h Pa
vem
ent a
nd
Stru
ctur
es
Cle
ar a
nd G
rub
Con
stru
ct A
cces
s R
oads
Gra
ding
(inc
. cut
an
d fil
l slo
pes)
Cha
nnel
Ex
cava
tion
Tren
chin
g an
d U
nder
grou
nd
Dra
inag
e
Util
ity T
renc
hing
Util
ity In
stal
latio
n
Subg
rade
pr
epar
atio
n
Base
Pav
ing
Pavi
ng
Saw
Cut
ting
Join
t Sea
ling
Grin
d/G
roov
e
Stru
ctur
e Ex
cava
tion
Erec
t Fal
sew
ork
Brid
ge/S
truct
ure
Con
stru
ctio
n
Rem
ove
Fals
ewor
k
Strip
ing
Mis
c. C
oncr
ete
Wor
k
Soun
d an
d R
etai
ning
Wal
ls
Plan
ting
and
Irrig
atio
n
Con
tract
or
Activ
ities
Trea
tmen
t BM
P C
onst
ruct
ion
Pile-Driving Operations X X X X X X Concrete Curing X X X X X X Material and Equipment Use Over Water
X X X X X X X X X X X X X X
Concrete Finishing X X X Structure Demolition Over Or Adjacent To Water X X X X
Table 7-1: Construction Site BMPs by Construction Activity 1
7 - 5 August 2010
7.1 SOIL STABILIZATION PRACTICES 1
Temporary soil stabilization practices consist of preparing the soil surface and applying 2 one of the BMPs shown in Table 7-2, or a combination thereof, to disturbed soil areas. 3
4
BMP Name
Scheduling
Preservation of Existing Vegetation
Hydraulic Mulch
Hydroseeding
Soil Binders
Fiber Mulch
Geotextiles, Plastic Covers, and Erosion Control Blankets/Mats
Temporary Concentrated Flow Conveyance Controls
Stream Bank Stabilization
Table 7-2: Temporary Soil Stabilization BMPs 5
7.1.1 SCHEDULING 6
Definition and Purpose 7
This BMP involves developing a schedule for every project that includes the sequencing 8 of construction activities with the implementation of construction site BMPs such as 9 temporary soil stabilization (erosion control) and temporary sediment control measures. 10 The purpose is to reduce the amount and duration of soil exposed to erosion by wind, 11 rain, runoff, and vehicle tracking, and to perform the construction activities and control 12 practices in accordance with the planned schedule. 13
Appropriate Applications 14
Construction sequencing should be scheduled to minimize land disturbance for all 15 projects during the rainy season. Appropriate BMPs should be implemented during both 16 rainy and non-rainy seasons. 17
Limitations 18
None identified. 19
Standards and Specifications 20
Developing a schedule and planning the project are the very first steps in an 21 effective stormwater program. The schedule should clearly show how the rainy 22
7 - 6 August 2010
season relates to soil-disturbing and re-stabilization activities. The construction 1 schedule should be incorporated into the Stormwater Pollution Prevention Plan 2 (SWPPP). 3
The schedule should include details on the rainy season implementation and 4 deployment of: 5
o Temporary soil stabilization BMPs 6
o Temporary sediment control BMPs 7
o Tracking control BMPs 8
o Wind erosion control BMPs 9
o Non-stormwater BMPs 10
o Waste management and materials pollution control BMPs 11
The schedule should also include dates for significant long-term operations or 12 activities that may have planned non-stormwater discharges such as de-13 watering, saw-cutting, grinding, drilling, boring, crushing, blasting, painting, 14 hydro-demolition, mortar mixing, bridge cleaning, etc. 15
Schedule work to minimize soil-disturbing activities during the rainy season. 16
o Develop the sequencing and timetable for the start and completion of 17 each item, such as site clearing and grubbing, grading, excavation, 18 paving, pouring foundations, installing utilities, etc., to minimize the active 19 construction area during the rainy season. 20
o Schedule major grading operations for the non-rainy season when 21 practical. 22
o Stabilize non-active areas within two days during wet season and seven 23 days during dry season, or one day prior to the onset of precipitation, 24 whichever occurs first. 25
o Monitor the weather forecast for rainfall. 26
o When rainfall is predicted, adjust the construction schedule to allow the 27 implementation of soil stabilization and sediment controls and sediment 28 treatment controls on all disturbed areas prior to the onset of rain. 29
Be prepared year-round to deploy soil stabilization and sediment control 30 practices. Erosion may be caused during dry seasons by unseasonal rainfall, 31 wind, and vehicle tracking. Keep the site stabilized year round, and retain and 32 maintain rainy-season sediment-trapping devices in operational condition. 33
Sequence trenching activities so that most open portions are closed before new 34 trenching begins. 35
7 - 7 August 2010
Incorporate staged seeding and the re-vegetation of graded slopes as work 1 progresses. 2
Consider scheduling when establishing permanent vegetation (appropriate 3 planting time for specified vegetation). 4
Apply permanent erosion control to areas deemed substantially complete during 5 the project’s defined seeding window. 6
Maintenance and Inspection 7
Verify that work is progressing in accordance with the schedule. If progress 8 deviates, take corrective actions. 9
Amend the schedule when changes are warranted. 10
The contractual specifications may require the annual submittal of a rainy season 11 implementation schedule. Amend the schedule prior to the rainy season to show 12 updated information on the deployment and implementation of construction site 13 BMPs. 14
7.1.2 PRESERVATION OF EXISTING VEGETATION 15
Definition and Purpose 16
The preservation of existing vegetation is the identification and protection of desirable 17 vegetation that provides erosion and sediment control benefits. The purpose of 18 preserving natural vegetation is to reduce erosion wherever practicable. Limiting site 19 disturbance is the single most effective method for reducing erosion. 20
Appropriate Applications 21
Preserve existing vegetation at areas on a site where no construction activity is 22 planned or where construction will occur at a later date. Specifications for the 23 preservation of existing vegetation can be found in the Federal Highway 24
Administration (FHWA) Standard Specifications for Construction of Roads and 25 Bridges on Federal Highway Projects, Section 201 (FP-03) (Standard 26 Specifications). On a year-round basis, temporary fencing should be placed 27 around areas not to be disturbed prior to the commencement of clearing and 28 grubbing operations or other soil-disturbing activities. 29
Clearing and grubbing operations should be staged to preserve existing 30 vegetation. 31
Limitations 32
The protection of existing vegetation requires planning and may limit the area available 33 for construction activities. 34
35
7 - 8 August 2010
Standards and Specifications 1
Timing 2
The preservation and fencing-off of existing vegetation should be provided 3 prior to the commencement of clearing and grubbing operations or other soil-4 disturbing activities in areas identified on the plans to be preserved, 5 especially on areas designated as environmentally sensitive. 6
The preservation of existing vegetation should conform to the approved 7 project schedule. 8
Design and Layout 9
Mark areas to be preserved with temporary fencing made of orange 10 polypropylene that is stabilized against ultraviolet (UV) light. The temporary 11 fencing should be at least 3.2 ft. tall and should have openings no larger than 12 2 in. by 2 in. 13
At the contractor’s discretion, fence posts should be either wood or metal, as 14 appropriate for the intended purpose. The post spacing and depth should be 15 adequate to completely support the fence in an upright position. 16
Minimize the disturbed areas by locating temporary roadways to avoid stands 17 of trees and shrubs and to follow existing contours to reduce cutting and 18 filling. 19
Consider the impact of grade changes to existing vegetation and the root 20 zone. 21
Installation 22
Construction materials, equipment storage, and parking areas should be 23 located where they will not cause root compaction. 24
Keep equipment away from trees to prevent trunk and root damage. 25
Maintain existing irrigation systems. 26
Employees and subcontractors should be instructed to honor protective 27 devices. 28
No heavy equipment, vehicular traffic, or storage piles of any construction 29 materials should be permitted within the drip line of any tree to be retained. 30
Removed trees should not be felled, pushed, or pulled into any retained 31 trees. 32
Fires should not be permitted within 100 ft. of the drip line of any retained 33 trees. Any fires should be of limited size and should be kept under continual 34
7 - 9 August 2010
surveillance. No toxic or construction materials (including paint, acid, nails, 1 gypsum board, chemicals, fuels, and lubricants) should be stored within 50 ft. 2 of the drip line of any retained trees, nor disposed of in any way that would 3 injure vegetation. 4
Trenching and Tunneling 5
Trenching should be as far away from tree trunks as possible, usually outside 6 of the tree drip line or canopy. Curve trenches around trees to avoid large 7 roots or root concentrations. If roots are encountered, consider tunneling 8 under them. When trenching and/or tunneling near or under trees to be 9 retained, tunnels should be at least 18 in. below the ground surface and not 10 below the tree center, to minimize impact on the roots. 11
Tree roots should not be left exposed to air; they should be covered with soil 12 as soon as possible, protected, and kept moistened with wet burlap or peat 13 moss until the tunnel and/or trench can be completed. 14
The ends of damaged or cut roots should be cut off smoothly. 15
Trenches and tunnels should be filled as soon as possible. Careful filling and 16 tamping will eliminate air spaces in the soil, which can damage roots. 17
Remove any trees intended for retention if those trees are damaged seriously 18 enough to affect their survival. If replacement is desired or required, the new 19 tree should be of a similar species, and at least 2 in. caliper, unless otherwise 20 required by the contract documents. 21
After all other work is complete, fences and barriers should be removed last. 22 This is because protected trees may be destroyed by carelessness during 23 final cleanup and landscaping. 24
Maintenance and Inspection 25
During construction, the limits of disturbance should remain clearly marked at 26 all times. Irrigation or maintenance of existing vegetation should conform to 27 the requirements in the landscaping plan. If damage to protected trees 28 occurs, the trees should be attended to by an arborist. 29
7.1.3 HYDRAULIC MULCH 30
Definition and Purpose 31
Hydraulic mulch consists of applying a mixture of shredded wood, plant, or pulp fiber, or 32 another bonded fiber matrix and a stabilizing emulsion or tackifier with hydroseeding 33 equipment, which temporarily protects exposed soil from erosion by raindrop impact or 34 wind. This is one of five temporary soil stabilization alternatives to consider. 35
36
7 - 10 August 2010
Appropriate Applications 1
Hydraulic mulch is applied to disturbed areas requiring temporary protection until 2 permanent vegetation is established or disturbed areas that must be re-3 disturbed, following an extended period of inactivity. 4
Limitations 5
Hydraulic mulches are generally short-lived (only last a part of a growing season) and 6 need 24 hours to dry before rainfall occurs, to be effective. Avoid use in areas where the 7 mulch would be incompatible with immediate future earthwork activities and would have 8 to be removed. 9
Standards and Specifications 10
Prior to application, roughen embankment and fill areas by rolling with a crimping 11 or punching-type roller or by track walking. Track walking should only be used 12 where other methods are impractical. 13
Hydraulic matrices require 24 hours to dry before rainfall occurs to be effective. 14
Avoid mulch overspray onto the traveled way, sidewalks, lined drainage 15 channels, and existing vegetation. 16
Hydraulic mulch should be applied in accordance with Standard Specification, 17 Section 625.08 (b), Hydraulic Method. 18
Materials for hydraulic mulches and hydraulic matrices should conform to 19 Standard Specification, Section 713.05. 20
Hydraulic Mulch 21
Wood fiber mulch is a component of hydraulic applications. It is typically 22 applied at the rate of 2,000 pounds per acre to 4,000 pounds per acre, with 0 23 percent to 5 percent by weight of a stabilizing emulsion or tackifier, such as 24 guar, psyllium, and acrylic copolymer, and applied as a slurry. This type of 25 mulch is manufactured from wood or wood waste from lumber mills or from 26 urban sources. 27
A hydraulic matrix is a combination of fiber mulch and a tackifier applied as a 28 slurry. It is typically applied at the rate of 1,500 pounds per acre, with 29 5 percent to 10 percent by weight of a stabilizing emulsion or tackifier such as 30 guar, psyllium, and acrylic copolymer. 31
Bonded Fiber Matrix (BFM) is a hydraulically-applied system of fibers and 32 adhesives that, upon drying, forms an erosion-resistant blanket that promotes 33 vegetation and prevents soil erosion. BFMs are typically applied at rates from 34 3,400 kilograms per hectare to 4,500 kilograms per hectare, based on the 35 manufacturer’s recommendation. The biodegradable BFM is composed of 36
7 - 11 August 2010
materials that are 100 percent biodegradable. The binder in the BFM should 1 also be biodegradable and should not dissolve or disperse upon re-wetting. 2 Typically, biodegradable BFMs should not be applied immediately before, 3 during, or after rainfall, if the soil is saturated. Depending on the product, 4 BFMs require 12 hours to 24 hours to dry to become effective. 5
Maintenance and Inspections 6
Maintain an unbroken, temporary mulched ground cover throughout the period of 7 construction when the soils are not being reworked. Inspect the ground cover 8 before expected rain storms, repair any damaged ground cover, and re-mulch 9 the exposed areas of bare soil. 10
After any rainfall event, the contractor is responsible for maintaining all slopes to 11 prevent erosion. 12
7.1.4 HYDROSEEDING 13
Definition and Purpose 14
Hydroseeding typically consists of applying a mixture of wood, plant or pulp fiber, seed, 15 fertilizer, and stabilizing emulsion with hydromulch equipment, which temporarily 16 protects exposed soils from erosion by water and wind. This is one of five temporary soil 17 stabilization alternatives to consider. 18
Appropriate Applications 19
Hydroseeding is applied on disturbed soil areas requiring temporary protection until 20 permanent vegetation is established, or disturbed soil areas that must be re-disturbed 21 following an extended period of inactivity. 22
Limitations 23
Hydroseeding may be used alone only when there is sufficient time in the season 24 to ensure adequate vegetation establishment and erosion control. Otherwise, 25 hydroseeding must be used in conjunction with a soil binder or heavier layer of 26 mulching such as wood, plant, or pulp fiber mulch. 27
Steep slopes are difficult to protect with temporary seeding. 28
Temporary seeding may not be appropriate in dry periods without supplemental 29 irrigation. 30
Temporary vegetation may have to be removed before permanent vegetation is 31 applied. 32
Temporary vegetation is not appropriate for short-term inactivity. 33
34
35
7 - 12 August 2010
Standards and Specifications 1
To select appropriate hydroseeding mixtures, an evaluation of site conditions should be 2 performed with respect to: 3
Soil conditions 4
Site topography 5
Season and climate 6
Vegetation types 7
Maintenance requirements 8
Sensitive adjacent areas 9
Water availability 10
Plans for permanent vegetation 11
The following steps should be followed for implementation: 12
Seed mix should comply with the Standard Specifications, Section 713.04, and 13 be of type to match the application and climate conditions in accordance with the 14 guidance provided by the Guam Division of Aquatic and Wildlife Resources 15 (DAWR) or the USDA, National Resources Conservation Service (NRCS). 16
Hydroseeding should be performed in accordance with Standard Specifications 17 625.06 or 625.07. 18
Prior to application, roughen the slope, fill area, or area to be seeded with the 19 furrows trending along the contours. Rolling with a crimping or punching-type 20 roller or track walking is required on all slopes prior to hydroseeding. Track 21 walking should only be used where other methods are impractical. 22
Apply a fiber mulch to keep seeds in place and to moderate soil moisture and 23 temperature until the seeds germinate and grow. Refer to Standard Specification 24 625.08. 25
Follow-up applications should be made as needed to cover weak spots and to 26 maintain adequate soil protection. 27
Avoid overspray onto the traveled way, sidewalks, lined drainage channels, and 28 existing vegetation. 29
Maintenance and Inspection 30
All seeded areas should be inspected for failures and re-seeded, fertilized, and 31 mulched within the planting season, using not less than half the original 32
7 - 13 August 2010
application rates. Any temporary re-vegetation efforts that do not provide 1 adequate cover must be reapplied. 2
After any rainfall event, the contractor is responsible for maintaining all slopes to 3 prevent erosion. 4
7.1.5 SOIL BINDERS 5
Definition and Purpose 6
Soil binders consist of applying and maintaining a soil stabilizer to exposed soil surfaces. 7 Soil binders are materials applied to the soil surface to temporarily prevent water-8 induced erosion of exposed soils on construction sites. Soil binders also provide 9 temporary dust, wind, and soil stabilization (erosion control) benefits. This is one of five 10 temporary soil stabilization alternatives to consider. 11
Appropriate Applications 12
Soil binders are typically applied to disturbed areas requiring short-term temporary 13 protection. Because soil binders can often be incorporated into the work, they may be a 14 good choice for areas where grading activities will soon resume, and as an application 15 on stockpiles to prevent water and wind erosion. 16
Limitations 17
Soil binders are temporary in nature and may need reapplication. 18
Soil binders require a minimum curing time until fully-effective, as prescribed by 19 the manufacturer, which may be 24 hours or longer. Soil binders may need 20 reapplication after a storm event. 21
Soil binders will generally experience spot failures during heavy rainfall events. If 22 runoff penetrates the soil at the top of a slope treated with a soil binder, it is likely 23 that the runoff will undercut the stabilized soil layer and discharge at a point 24 farther down slope. 25
Soil binders do not hold up to pedestrian or vehicular traffic across treated areas. 26
Soil binders may not penetrate soil surfaces made up primarily of silt and clay, 27 particularly when compacted. 28
Some soil binders may not perform well with low relative humidity. Under rainy 29 conditions, some agents may become slippery or leach out of the soil. 30
Standards and Specifications 31
General Considerations 32
Site-specific soil types will dictate the appropriate soil binders to be used. 33 Refer to the soil binder supplier’s literature for proper soil types and 34
7 - 14 August 2010
application. 1
A soil binder must be environmentally benign (non-toxic to plant and animal 2 life), easy to apply, easy to maintain, economical, and should not stain paved 3 or painted surfaces. 4
Some soil binders are not compatible with existing vegetation. Check the 5 supplier’s guidelines for applications. 6
Avoid overspray onto the traveled way, sidewalks, lined drainage channels, 7 and existing vegetation. 8
Soil Binders Applications 9
After selecting an appropriate soil binder, the untreated soil surface must be prepared 10 before applying the soil binder. The untreated soil surface must contain sufficient 11 moisture to assist the agent in achieving uniform distribution. In general, the following 12 steps should be followed: 13
Follow the manufacturer’s recommendations for the application rates, pre-wetting 14 of the application area, and cleaning of equipment after use. 15
Prior to application, roughen embankment and fill areas by rolling with a crimping 16 or punching-type roller or by track walking. Track walking should only be used 17 where rolling is impractical. 18
Consider the drying time for the selected soil binder and apply with sufficient time 19 before anticipated rainfall. Soil binders should not be applied during or 20 immediately before rainfall. 21
More than one treatment is often necessary, although the second treatment may 22 be diluted or have a lower application rate. 23
Generally, soil binders require a minimum curing time of 24 hours before they are 24 fully effective. Refer to the manufacturer’s instructions for specific cure times. 25
For Liquid Agents 26
Crown or slope ground to avoid ponding. 27
Uniformly pre-wet ground at 0.03 gallons per square yard of area, or 28 according to the manufacturer’s recommendations. 29
Apply the solution under pressure. Overlap the solution 6 in. to 12 in. 30
Allow the treated area to cure for the time recommended by the 31 manufacturer, typically at least 24 hours. 32
In low humidity, reactivate chemicals by re-wetting with water at 0.1 gallon 33 per square yard area to 0.2 gallon per square yard. 34
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Selecting a Soil Binder 1
The properties of common soil binders used for erosion control are provided in Table 7-2 5. Factors to consider when selecting a soil binder include the following: 3
Suitability to situation: Consider where the soil binder will be applied; determine if 4 it needs a high resistance to leaching or abrasion, and whether it needs to be 5 compatible with any existing vegetation. Determine the length of time soil 6 stabilization will be needed and if the soil binder will be placed in an area where it 7 will degrade rapidly. In general, slope steepness is not a discriminating factor for 8 the listed soil binders. 9
Soil types and surface materials: Fines and moisture content are key properties 10 of surface materials. Consider a soil binder’s ability to penetrate, its likelihood of 11 leaching, and its ability to form a surface crust on the surface materials. 12
Frequency of application: The frequency of application can be affected by 13 subgrade conditions, surface type, climate, and maintenance schedule. Frequent 14 applications could lead to high costs. Application frequency may be minimized if 15 the soil binder has good penetration, low evaporation, and good longevity. Also 16 consider that frequent application will require frequent equipment cleanup. 17
After considering the above factors, the soil binders in Table 7-5 will be generally 18 appropriate, as follows: 19
Plant Material Based (Short-Lived) 20
Guar: Guar is a nontoxic, biodegradable, natural galactomannan-based 21 hydrocolloid treated with dispersant agents for easy field mixing. It should be 22 diluted at the rate of 1 pound to 5 pounds per 100 gallons of water, depending on 23 the capacity of the application machine. The recommended minimum application 24 rates are as follows: 25
26 Slope (V:H)
Flat 1:4 1:3 1:2 1:1
Lb/Acres 40 45 50 60 70
Table 7-3: Application Rates for Guar Stabilizer 27
Psyllium: Psyllium is composed of the finely-ground mucilloid coating of 28 plantago seeds that is applied as a dry powder or in a wet slurry to the surface of 29 the soil. It dries to form a firm but re-wettable membrane that binds soil particles 30 together but permits germination and seed growth. Psyllium requires 2 hours to 31 8 hours of drying time. Psyllium should be applied at a rate of 80 pounds per acre 32
7 - 16 August 2010
to 200 pounds per acre, with enough water in the solution to allow for a uniform 1 slurry flow. 2
Starch: Starch is non-ionic, cold-water soluble (pre-gelatinized), granular 3 cornstarch. The material is mixed with water and applied at the rate of 150 4 pounds per acre. The approximate drying time is 9 hours to 12 hours. 5
Plant Material Based (Long-Lived) 6
Pitch and Rosin Emulsion: Generally, a non-ionic pitch and rosin emulsion has 7 a minimum solids content of 48 percent. The rosin should be a minimum of 26 8 percent of the total solids content. The soil stabilizer should be non-corrosive, 9 water-dilutable, emulsion that upon application, cures to a water-insoluble 10 binding and cementing agent. For soil erosion control applications, the emulsion 11 is diluted and should be applied as follows: 12
o For clayey soil: Five parts water to 1 part emulsion 13
o For sandy soil: Zero parts water to 1 part emulsion 14
The application can be by water truck or hydraulic seeder, with the 15 emulsion/product mixture applied at the rate specified by the manufacturer. The 16 approximate drying time is 19 hours to 24 hours. 17
Polymeric Emulsion Blends 18
Acrylic Copolymers and Polymers: Polymeric soil stabilizers should consist of 19 a liquid or solid polymer or copolymer with an acrylic base that contains a 20 minimum of 55 percent solids. The polymeric compound should be handled and 21 mixed in a manner that will not cause foaming, or it should contain an anti-22 foaming agent. The polymeric emulsion should not exceed its shelf life or 23 expiration date; manufacturers should provide the expiration date. Polymeric soil 24 stabilizer should be readily miscible in water, non-injurious to seed or animal life, 25 and non-flammable; provide surface soil stabilization for various soil types 26 without totally inhibiting water infiltration; and not re-emulsify when cured. The 27 applied compound should air cure within a maximum of 36 hours to 48 hours. 28 Liquid copolymer should be diluted at a rate of 10 parts water to 1 part polymer, 29 and applied to soil at a rate of 11,000 liters per hectare (1,175 gallons per acre). 30
Liquid Polymers of Methacrylates and Acrylates: This material consists of a 31 tackifier/sealer that is a liquid polymer of methacrylates and acrylates. It is an 32 aqueous 100 percent acrylic emulsion blend of 40 percent solids by volume that 33 is free from styrene, acetate, vinyl, ethoxylated surfactants, or silicates. For soil 34 stabilization applications, it is diluted with water in accordance with the 35 manufacturer’s recommendations and applied with a hydraulic seeder at the rate 36
7 - 17 August 2010
of 190 liters per hectare (20 gallons per acre). The drying time is 12 hours to 18 1 hours after application. 2
Copolymers of Sodium Acrylates and Acrylamides: These materials are 3 nontoxic, dry powders that are copolymers of sodium acrylate and acrylamide. 4 They are mixed with water and applied to the soil surface for erosion control at 5 rates that are determined by slope gradient: 6
7 Slope Gradient (V:H) Lbs/Acres
Flat to 1:5 3-5
1:5 to 1:3 5-10
1:2 to 1:1 10-20
Table 7-4: Application Rates for Copolymers 8
Polyacrylamide and Copolymer of Acrylamide: Linear copolymer 9 polyacrylamide is packaged as a dry-flowable solid. When used as a stand-alone 10 stabilizer, it is diluted at a rate of 1 pound per 100 gallons of water and applied at 11 the rate of 15 pounds per acre. 12
Hydrocolloid Polymers: Hydrocolloid polymers are various combinations of dry- 13 flowable polyacrylamides, copolymers, and hydrocolloid polymers that are mixed 14 with water and applied to the soil surface at rates of 60 kilograms per hectare to 15 70 kilograms per hectare (53 pounds per acre to 62 pounds per acre). Drying 16 times are 0 hours to 4 hours. 17
Cementitious-Based Binders 18
Gypsum: This is a formulated gypsum-based product that readily mixes with 19 water and mulch to form a thin protective crust on the soil surface. It is composed 20 of high-purity gypsum that is ground, calcined, and processed into calcium 21 sulfate hemihydrate with a minimum purity of 86 percent. It is mixed in a 22 hydraulic seeder and applied at rates of 4,000 pounds per acre to 12,000 pounds 23 per acre. The drying time is 4 hours to 8 hours. 24
Maintenance and Inspection 25
Reapplying the selected soil binder may be needed for proper maintenance. 26 High-traffic areas should be inspected daily, and lower traffic areas should be 27 inspected weekly. 28
After any rainfall event, the contractor is responsible for maintaining all slopes to 29 prevent erosion. 30
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Maintain an unbroken, temporary stabilized area while disturbed soil areas are 1 non-active. 2
Repair any damaged, stabilized area and reapply soil binder to exposed areas. 3
4 Properties of Soil Binders for Erosion Control
Chemicals
Plant-Material Based (Short-
Lived)
Plant-Material Based (Long-
Lived)
Polymeric Emulsion
Blends
Cementitious-Based Binders
Relative Cost Low Low Low Low Resistance to Leaching
High High Low to Moderate Moderate
Resistance to Abrasion
Moderate Low Moderate to High Moderate to High
Longevity Short to Medium Medium Medium to Long Medium Minimum Curing Time Before Rain
9 to 18 hours 19 to 24 hours 0 to 24 hours 4 to 8 hours
Compatibility with Existing Vegetation
Good Poor Poor Poor
Mode of Degradation
Biodegradable Biodegradable Photodegradable/ Chemically Degradable
Photodegradable/ Chemically Degradable
Labor Intensive No No No No Specialized Application Equipment
Water Truck or Hydraulic Mulcher
Water Truck or Hydraulic Mulcher
Water Truck or Hydraulic Mulcher
Water Truck or Hydraulic Mulcher
Liquid/Powder Powder Liquid Liquid/Powder Powder Surface Crusting Yes, but
dissolves on re-wetting
Yes Yes, but dissolves on
rewetting
Yes
Cleanup Water Water Water Water Erosion Control Application Rate
Varies (1) Varies (1) Varies (1) 4,500 to 13,500 kg/ha
(1) See application rate Tables 7-3 and 7-4. 5
Table 7-5: Properties of Soil Binders for Erosion Control 6
7.1.6 FIBER MULCH 7
Definition and Purpose 8
Using fiber mulch involves placing a uniform layer of wood, vegetable, or pulp fibers, and 9 incorporating it into the soil with a studded roller or anchoring it with a stabilizing 10 emulsion. This is one of five temporary soil stabilization alternatives to consider. 11
Appropriate Applications 12
Fiber mulch is typically used for soil stabilization as a temporary surface cover on 13 disturbed areas until soils can be prepared for re-vegetation and permanent 14 vegetation is established. 15
7 - 19 August 2010
Also, fiber mulch is typically used in combination with temporary and/or 1 permanent seeding strategies to enhance plant establishment. 2
Limitations 3
The availability of erosion control contractors and fiber may be limited prior to the 4 rainy season due to high demand. 5
There is a potential for the introduction of weed-seed and unwanted plant 6 material in vegetable fiber such as grass or straw mulch. 7
When blowers are used to apply fiber mulch, the treatment areas must be within 8 150 ft. of a road or surface capable of supporting trucks. 9
Fiber mulch applied by hand is more time intensive and potentially costly. 10
Fiber mulch may have to be removed prior to permanent seeding or soil 11 stabilization. 12
The “punching” of fiber mulch does not work in sandy soils. 13
Standards and Specifications 14
All materials should conform to the Standard Specifications. A tackifier is the 15 preferred method for anchoring fiber mulch to the soil on slopes. 16
Crimping, punch roller-type rollers, or track walking may also be used to 17 incorporate fiber mulch into the soil on slopes. Track walking should only be used 18 where other methods are impractical. 19
Avoid placing mulch onto the traveled way, sidewalks, lined drainage channels, 20 sound walls, and existing vegetation. 21
Fiber mulch with tackifier should not be applied during or immediately before 22 rainfall. 23
Application Procedures 24
Apply loose fiber at a minimum rate of 3,200 pounds per acre or, as indicated in 25 the Standard Specification 625.08. 26
If stabilizing emulsion will be used to anchor the fiber mulch, roughen the 27 embankment or fill areas by rolling with a crimping or punching-type roller, or by 28 track walking before placing the mulch. Track walking should only be used where 29 rolling is impractical. 30
The fiber mulch must be evenly distributed on the soil surface. 31
Anchor the mulch in place by using a tackifier or by punching it into the soil 32 mechanically (incorporating). 33
7 - 20 August 2010
A tackifier acts to glue the mulch fibers together and to the soil surface. The 1 tackifier should be selected based on its longevity and ability to hold the fibers in 2 place. 3
A tackifier is typically applied at a rate of 125 pounds per acre. In windy 4 conditions, the rates are typically 178 pounds per acre. 5
Methods for holding the fiber mulch in place depend upon the slope steepness, 6 accessibility, soil conditions, and longevity. If the selected method is the 7 incorporation of fiber mulch into the soil, then perform the following: 8
o Applying and incorporating fiber should follow the requirements of 9 Standard Specification 625.08. 10
o On small areas, a spade or shovel can be used. 11
o On slopes with soils that are stable enough and of sufficient gradient to 12 safely support construction equipment without contributing to compaction 13 and instability problems, mulch can be punched into the ground using a 14 knife blade roller or a straight-bladed coulter, known commercially as a 15 crimper. 16
o On small areas and/or steep slopes, straw can also be held in place using 17 plastic netting or jute. The netting should be held in place using 11-gauge 18 wire staples, geotextile pins, or wooden stakes. 19
Maintenance and Inspections 20
The key consideration in maintenance and inspection is that the mulch needs to 21 last long enough to achieve erosion control objectives. 22
Maintain an unbroken, temporary mulched ground cover while disturbed soil 23 areas are non-active. Repair any damaged ground cover and re-mulch exposed 24 areas. 25
The reapplication of fiber mulch and tackifier may be required to maintain 26 effective soil stabilization over disturbed areas and slopes. 27
After any rainfall event, the contractor is responsible for maintaining all slopes to 28 prevent erosion 29
7.1.7 GEOTEXTILES, PLASTIC COVERS, AND EROSION CONTROL 30 BLANKETS/MATS 31
Definition and Purpose 32
This BMP involves the placement of geotextiles, mats, plastic covers, or erosion control 33 blankets to stabilize disturbed soil areas and protect soils from erosion by wind or water. 34 This is one of the five temporary soil stabilization alternatives to consider. 35
7 - 21 August 2010
Appropriate Applications 1
These measures are used when disturbed soils may be particularly difficult to stabilize, 2 including the following situations: 3
Steep slopes, generally steeper than 1:3 (V:H) 4
Slopes where the erosion potential is high 5
Slopes and disturbed soils where plants are slow to develop 6
Slopes and disturbed soils where mulch must be anchored 7
Channels with flows exceeding 3.3 ft. per second 8
Channels to be vegetated 9
Stockpiles 10
Slopes adjacent to environmentally sensitive areas 11
Limitations 12
Blankets and mats are more expensive than other erosion control measures due 13 to labor and material costs. This usually limits their application to areas 14 inaccessible to hydraulic equipment or where other measures are not applicable, 15 such as channels. 16
Blankets and mats are generally not suitable for excessively rocky sites or areas 17 where final vegetation will be mowed (because staples and netting can get 18 caught in the mower). 19
Blankets and mats must be removed and disposed of prior to the application of 20 permanent soil stabilization measures. 21
Plastic sheet is easily vandalized, easily torn, and photodegradable, and must be 22 disposed of at a landfill. 23
Plastic results in 100 percent runoff, which may cause serious erosion problems 24 in the areas receiving increased flows. 25
The use of plastic should be limited to covering stockpiles, or very small graded 26 areas for short periods of time (such as through one imminent storm event), until 27 alternative measures, such as seeding and mulching, may be installed. 28
Geotextile, mats, plastic covers, and erosion control blankets have maximum 29 flow rate limitations; consult the manufacturer for proper selection. 30
Standards and Specifications 31
There are many types of erosion control blankets and mats, and selection of the 32 appropriate type should be based on the specific type of application and site conditions. 33
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Selections made by the contractor must be approved with certification compliance made 1 in accordance with the Standard Specifications. 2
Geotextile 3
Materials should be woven polypropylene fabric with a minimum thickness of 4 0.06 in.; a minimum width of 12 ft.; and a minimum tensile strength of 0.67 5 kiloNewtons (warp), 0.36 kiloNewtons (fill), in conformance with the 6 requirements in the American Society for Testing and Materials (ASTM) 7 Designation D4491. The fabric should have a UV stability of 70 percent in 8 conformance with the requirements in ASTM Designation D4355. Geotextile 9 blankets should be secured in place with wire staples or sandbags and by 10 keying into the tops of slopes and edges to prevent the infiltration of surface 11 waters under geotextile. Staples should be made of 0.12 in. of steel wire and 12 U-shaped, with 8-in. legs and a 2-in. crown. 13
Geotextile materials may be reused if they are suitable for their intended use. 14
Plastic Covers 15
Plastic sheeting should have a minimum thickness of 6 mils and should be 16 keyed in at the top of the slope and firmly placed with sandbags or other 17 weights placed no more than 10 ft. apart. Seams are typically taped or 18 weighted down their entire length, and there should be at least a 12-in. to a 19 24-in. overlap of all seams. Edges should be embedded a minimum of 6 in. in 20 soil. 21
All sheeting should be inspected periodically after installation and after 22 significant rainstorms to check for erosion, undermining, and anchorage 23 failure. Any failure should be repaired immediately. If washout or breakages 24 occur, the material should be re-installed after repairing damage to the slope. 25
Erosion Control Blankets/Mats 26
Biodegradable Rolled Erosion Control Products (RECPs) are typically 27 comprised of jute fibers, curled wood fibers, straw, coconut fibers, or a 28 combination of these materials. For an RECP to be considered 100 percent 29 biodegradable, the netting, sewing, or adhesive system that holds the 30 biodegradable mulch fibers together must also be biodegradable. 31
Jute is a natural fiber that is made into yarn, which is loosely woven into a 32 biodegradable mesh. It is designed to be used in conjunction with vegetation 33 and has a longevity of approximately one year. The material is supplied in 34 rolled strips, which should be secured to soil with U-shaped staples or stakes 35 in accordance with the manufacturer’s recommendations. 36
7 - 23 August 2010
Excelsior (curled wood fiber) blanket materials should consist of machine- 1 produced mats of curled wood excelsior with 80 percent of the fiber 6 in. or 2 longer. The excelsior blanket should be of consistent thickness. The wood 3 fiber should be evenly distributed over the entire area of the blanket. The top 4 surface of the blanket should be covered with a photodegradable extruded 5 plastic mesh. The blanket should be smolder resistant without the use of 6 chemical additives and nontoxic and non-injurious to plant and animal life. An 7 excelsior blanket should be furnished in rolled strips, a minimum of 48 in. 8 wide, and should have an average weight of 12 pounds per square foot, ± 10 9 percent, at the time of manufacture. Excelsior blankets should be secured in 10 place with wire staples. Staples should be made of 0.12-in. steel wire U-11 shaped, with 8-in. legs and a 2-in. crown. 12
Straw blanket should be machine-produced mats of straws with lightweight 13 biodegradable netting top layers. The straw should be attached to the netting 14 with biodegradable threads or glue strips. The straw blanket should be of 15 consistent thickness. The straw should be evenly distributed over the entire 16 area of the blanket. Straw blanket should be furnished in rolled strips a 17 minimum of 6.5 ft. wide, a minimum of 80 ft. long, and a minimum of 6.4 18 pounds per square foot. Blankets should be secured to the ground using wire 19 staples. Staples should be made of 0.12-in. steel wires and U-shaped, with 8-20 in. legs and a 2-in. crown. 21
Wood fiber blanket is composed of biodegradable fiber mulch, with extruded 22 plastic netting held together with adhesives. The material is designed to 23 enhance re-vegetation. The material is furnished in rolled strips, which should 24 be secured to the ground with U-shaped staples or stakes in accordance with 25 the manufacturer’s recommendations. 26
Coconut fiber blanket should be machine-produced mats of 100 percent 27 coconut fiber with biodegradable netting on the top and bottom. The coconut 28 fiber should be attached to the netting with biodegradable thread or glue 29 strips. The coconut blanket should be of consistent thickness. The coconut 30 fiber should be evenly distributed over the entire area of the blanket. The 31 coconut fiber blanket should be furnished as rolled strips a minimum of 6.5 ft. 32 wide, 80 ft. long, and 6.4 pounds per square foot. Coconut fiber blankets 33 should be secured in place with wire staples. Staples should be made of 34 0.12-in. steel wire and U-shaped, with 8-in. legs and a 2-in. crown. 35
Coconut fiber mesh is a thin permeable membrane made from coconut or 36 corn fiber that is spun into yarn and woven into a biodegradable mat. It is 37 designed to be used in conjunction with vegetation and typically has a 38 longevity of several years. The material is supplied in rolled strips, which 39
7 - 24 August 2010
should be secured to the soil with U-shaped staples or stakes in accordance 1 with the manufacturer’s recommendations. 2
Straw coconut fiber blankets should be machine-produced mats of 70 3 percent straw and 30 percent coconut fiber with a biodegradable netting top 4 layer and a biodegradable bottom net. The straw and coconut fiber should be 5 attached to the netting with biodegradable thread or glue strips. The straw 6 coconut fiber blankets should be of consistent thickness. The straw coconut 7 fiber should be evenly distributed over the entire area of the blanket. Straw 8 coconut fiber blankets should be furnished in rolled strips a minimum of 6.5 ft. 9 wide, 80 ft. long, and 6.4 pounds per square foot. Straw coconut fiber 10 blankets should be secured in place with wire staples. Staples should be 11 made of 0.12-in. steel wires and U-shaped, with 8-in. legs and a 2-in. crown. 12
Non-biodegradable RECPs are typically composed of polypropylene, 13 polyethylene, nylon, or synthetic fibers. In some cases, a combination of 14 biodegradable and synthetic fibers is used to construct the RECP. Netting is 15 used to hold these fibers together and is also typically non-biodegradable. 16
Plastic netting is a lightweight biaxial-oriented netting designed for securing 17 loose mulches like straw to soil surfaces to establish vegetation. The netting 18 is photo-biodegradable. The netting is supplied in rolled strips, which should 19 be secured with U-shaped staples or stakes in accordance with the 20 manufacturer’s recommendations. 21
Plastic mesh is an open-weave geotextile that is composed of an extruded 22 synthetic fiber woven into a mesh with an opening size of less than 0.2 in. It is 23 used with re-vegetation or may be used to secure loose fibers to the ground, 24 such as straw. The material is supplied in rolled strips, which should be 25 secured to the soil with U-shaped staples or stakes in accordance with the 26 manufacturer’s recommendations. 27
Synthetic fibers with netting form a mat composed of durable synthetic 28 fibers treated to resist chemicals and UV light. The mat is a dense, three-29 dimensional mesh of synthetic fibers (typically polyolefin) stitched between 30 two polypropylene nets. The mat is designed to be re-vegetated and provide 31 a permanent composite system of soils, roots, and geomatrix. The material is 32 furnished in rolled strips, which should be secured with U-shaped staples or 33 stakes in accordance with the manufacturer’s recommendations. 34
Bonded synthetic fibers consist of three-dimensional geomatrix nylon (or 35 other synthetic) matting. Typically, the matting has more than 90 percent 36 open area, which facilitates root growth. It can reinforce roots, anchor 37 vegetation, and protect against hydraulic lift and shear forces created by 38 high-volume discharges. It can be installed over prepared soil, followed by 39
7 - 25 August 2010
seeding into the mat. Once vegetated, it becomes an invisible composite 1 system of soils, roots, and geomatrix. The material is furnished in rolled 2 strips, which should be secured with U-shaped staples or stakes in 3 accordance with the manufacturer’s recommendations. 4
Combination synthetic and biodegradable RECPs consist of 5 biodegradable fibers, such as wood fibers or coconut fiber, with a heavy 6 polypropylene net stitched to the top and a high-strength continuous filament 7 geomatrix or net stitched to the bottom. The material is designed to enhance 8 re-vegetation. The material is furnished in rolled strips, which should be 9 secured with U-shaped staples or stakes in accordance with the 10 manufacturer’s recommendations. 11
Site Preparation 12
Proper site preparation is essential to ensure complete contact of the blanket or 13 matting with the soil grade, and shape the area of installation. 14
Remove all rocks, clods, vegetation, or other obstructions so that the installed 15 blankets or mats will have complete direct contact with soil. 16
Prepare the seed bed by loosening 2 in. to 3 in. of soil. 17
Seeding 18
Seed the area before blanket installation for erosion control and re-vegetation. Seeding 19 after mat installation is often specified for turf reinforcement application. When seeding 20 prior to blanket installation, all check slots and other areas disturbed during installation 21 must be re-seeded. Where soil filing is specified, seed the matting and the entire 22 disturbed area after installation and prior to filling the mat with soil. 23
Anchoring 24
U-shaped wire staples, metal geotextile stake pins, or triangular wooden stakes 25 can be used to anchor mats and blankets to the ground. 26
Staples should be made of 0.12-in. steel wire and U-shaped, with 8-in. legs and a 27 2-in. crown. 28
Metal stake pins should be 0.188-in. diameter steel with a 1.5-in. steel washer at 29 the head of the pin. 30
Wire staples and metal stakes should be driven flush to the soil surface. 31
All anchors should be 6 in. to 8 in. long and have sufficient ground penetration to 32 resist pullout. Longer anchors may be required for loose soils. 33
7 - 26 August 2010
Installation on Slopes 1
Installations should be in accordance with the manufacturer’s recommendations. In 2 general, these will be as follows: 3
Begin at the top of the slope, and anchor the blanket in 6-in. deep by 6-in. wide 4 trench. Backfill the trench and tamp the earth gently. 5
Unroll the blanket down-slope in the direction of water flow. 6
Overlap the edges of the adjacent, parallel roll 2 in. to 3 in., and staple it every 7 3 ft. 8
When blankets must be spliced, place the blankets end-over-end (shingle style), 9 with a 6-in. overlap. Staple through the overlap area, approximately 12 in. apart. 10
Temporary Soil Stabilization Removal 11
When no longer required for work, temporary soil stabilization should become the 12 property of the contractor. Temporary soil stabilization removed from the site of the work 13 should be disposed of outside of highway rights-of-way. 14
Maintenance and Inspection 15
Areas treated with temporary soil stabilization should be inspected weekly and 16 after each heavy rain. Areas treated with temporary soil stabilizations should be 17 maintained to provide adequate erosion control. Temporary soil stabilization 18 should be reapplied or replaced on exposed soils when the area becomes 19 exposed or exhibits visible erosion. 20
Installation should be inspected after significant rainstorms to check for erosion 21 and undermining. Any failures should be repaired immediately. 22
If washout or breakage occurs, re-install the material after repairing the damage 23 to the slope of the channel. 24
7.1.8 TEMPORARY CONCETRATED FLOW CONVEYANCE CONTROLS 25
Temporary concentrated flow conveyance controls consist of a system of measures or 26 BMPs that are used alone or in combination to intercept, divert, convey, and discharge 27 concentrated flows with a minimum of soil erosion, both on-site and downstream (off-28 site). Temporary concentrated flow conveyance controls may be required to direct run-29 on around or through the project in a non-erodible fashion. Temporary concentrated flow 30 conveyance controls include the following BMPs: 31
Earth dikes, drainage swales, and lined ditches 32
Outlet protection/velocity dissipation devices 33
Slope drains 34
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7.1.8.1 EARTH DIKES, DRAINAGE SWALES, AND LINED DITCHES 1
Definition and Purpose 2
These are structures that intercept, divert, and convey surface run-on, generally sheet 3 flow, to prevent erosion. 4
Appropriate Applications 5
Earth dikes, drainage swales, and lined ditches may be used to: 6
o Convey surface runoff down sloping land 7
o Intercept and divert runoff to avoid flow over sloped surfaces 8
o Divert and direct runoff toward a stabilized watercourse, drainage pipe, or 9 channel 10
o Intercept runoff from paved surfaces 11
Earth dikes, drainage swales, and lined ditches also may be used: 12
o Below steep grades where runoff begins to concentrate 13
o Along roadways and facility improvements subject to flood drainage 14
o At the top of slopes to divert run-on from adjacent or undisturbed slopes 15
o At bottom and mid-slope locations to intercept sheet flow and convey 16 concentrated flows 17
This BMP may be implemented on a project-by-project basis with other BMPs, 18 when determined necessary and feasible. 19
Limitations 20
Earth dikes, drainage swales, and lined ditches are not suitable as sediment-21 trapping devices. 22
It may be necessary to use other soil stabilization and sediment controls, such as 23 check dams, plastics, and blankets, to prevent scour and erosion in newly-24 graded dikes, swales, and ditches. 25
Standards and Specifications 26
Care must be applied to correctly size and locate earth dikes, drainage swales, 27 and lined ditches. Excessively steep, unlined dikes and swales are subject to 28 erosion and gully formation. 29
Conveyances should be stabilized. 30
Use a lined ditch for high flow velocities. 31
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Select flow velocity based on careful evaluation of the risks due to erosion of the 1 measure, soil types, overtopping, flow back-ups, washout, and drainage flow 2 patterns for each project site. 3
Compact any fills to prevent unequal settlement. 4
Do not divert runoff from the highway right-of-way onto other property. 5
When possible, install and utilize permanent dikes, swales, and ditches early in 6 the construction process. 7
Provide stabilized outlets. 8
Maintenance and Inspections 9
Inspect temporary measures prior to the rainy season, after significant rainfall 10 events, and regularly (approximately once per week) during the rainy season. 11
Inspect ditches and berms for washouts. Replace lost riprap, damaged linings, or 12 soil stabilizers as needed. 13
Inspect channel linings, embankments, and beds of ditches and berms for 14 erosion and the accumulation of debris and sediment. Remove debris and 15 sediment, and repair linings and embankments as needed. 16
Temporary conveyances should be completely removed as soon as the 17 surrounding drainage area has been stabilized, or at the completion of 18 construction. 19
7.1.8.2 OUTLET PROTECTION/VELOCITY DISSIPATION DEVICES 20
Definition and Purpose 21
These devices are placed at pipe outlets to prevent scour and reduce the velocity and/or 22 energy of stormwater flows. 23
Appropriate Applications 24
Outlets of pipes, drains, culverts, slope drains, diversion ditches, swales, 25 conduits, or channels 26
Outlets located at the bottom of mild to steep slopes 27
Discharge outlets that carry continuous flows of water 28
Outlets subject to short, intense flows of water, such as flash floods 29
Points where lined conveyances discharge to unlined conveyances 30
This BMP may be implemented on a project-by-project basis with other BMPs 31 when determined necessary and feasible. 32
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Limitations 1
Loose rock may have stones washed away during high flows. 2
If there is not adequate under-drainage and water builds up behind grouted 3 riprap, it may cause the grouted riprap to break up due to the resulting 4 hydrostatic pressure. 5
Standards and Specifications 6
There are many types of energy dissipaters. See FHWA publication HEC-14 for 7 details on types, applications, and design guidelines. 8
Install riprap, grouted riprap, or concrete apron at the selected outlet. Riprap 9 aprons are best suited for temporary use during construction. 10
Carefully place riprap to avoid damaging the filter fabric. 11
For proper operation of apron: 12
o Align apron with receiving stream and keep straight throughout its length. 13 If a curve is needed to fit site conditions, place it in the upper section of 14 apron. 15
o If the size of the apron riprap is large, protect underlying filter fabric with a 16 gravel blanket. 17
Outlets on slopes steeper than 10 percent should have additional protection. 18
Maintenance and Inspection 19
Inspect temporary measures prior to the rainy season, after rainfall events, and 20 regularly (approximately once per week) during the rainy season. 21
Inspect the apron for displacement of the riprap and/or damage to the underlying 22 fabric. Repair fabric and replace riprap that has washed away. 23
Inspect for scour beneath the riprap and around the outlet. Repair damage to 24 slopes or underlying filter fabric immediately. 25
Temporary devices should be completely removed as soon as the surrounding 26 drainage area has been stabilized, or at the completion of construction. 27
7.1.8.3 SLOPE DRAINS 28
Definition and Purpose 29
A slope drain is a pipe used to intercept and direct surface runoff or groundwater into a 30 stabilized watercourse, trapping device, or stabilized area. Slope drains are used with 31 lined ditches to intercept and direct surface flow away from slope areas to protect cut or 32 fill slopes. 33
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Appropriate Applications 1
Slope drains may be used on construction sites where slopes may be eroded by 2 surface runoff. 3
This BMP may be implemented on a project-by-project basis with other BMPs, 4 when determined necessary and feasible. 5
Limitations 6
Severe erosion may result when slope drains fail by overtopping, piping, or pipe 7 separation. 8
Standards and Specifications 9
When using slope drains, limit the drainage area to 10 acres per pipe. For larger 10 areas, use a rock-lined channel or a series of pipes. 11
The maximum slope is generally limited to 1:2 (V:H), as energy dissipation below 12 steeper slopes is difficult. 13
Direct surface runoff to slope drains with interceptor dikes. 14
Slope drains can be placed on or buried underneath the slope surface. 15
Recommended materials are PVC, ABS, or comparable pipe. 16
When installing slope drains: 17
o Install slope drains perpendicular to slope contours. 18
o Compact soil around and under the entrance, outlet, and along the pipe’s 19 length. 20
o Securely anchor and stabilize pipe and appurtenances into soil. 21
o Check to ensure that pipe connections are watertight. 22
o Protect the area around the inlet with filter cloth. Protect the outlet with 23 riprap or another energy-dissipation device. For high-energy discharges, 24 reinforce riprap with concrete or use a reinforced-concrete device. 25
o Protect the inlet and outlet of slope drains; use standard flared-end 26 sections at the entrance and exit for pipe slope drains 12 in. and larger. 27
Maintenance and Inspection 28
Inspect before and after each rain storm and twice monthly until the tributary 29 drainage area has been stabilized. Follow routine inspection procedures for inlets 30 thereafter. 31
Inspect the outlet for erosion and downstream scour. If eroded, repair the 32 damage and install additional energy dissipation measures. If downstream scour 33
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is occurring, it may be necessary to reduce flows being discharged into the 1 channel unless other preventive measures are implemented. 2
Inspect slope drainage for accumulations of debris and sediment. 3
Remove built-up sediment from entrances, outlets, and within drains as required. 4
Make sure water is not ponding onto inappropriate areas (e.g., active traffic 5 lanes, material storage areas, etc.). 6
7.1.9 STREAMBANK STABILIZATION 7
Definition and Purpose 8
Drainage systems, including the stream channel, streambank, and associated riparian 9 areas, are dynamic and sensitive ecosystems that respond to changes in land use 10 activity. Streambank and channel disturbance resulting from construction activities can 11 increase the stream’s sediment load, which can cause channel erosion or sedimentation 12 and have adverse affects on the biotic system. BMPs can reduce the discharge of 13 sediment and other pollutants and minimize the impact of construction activities on 14 watercourses. 15
Appropriate Applications 16
These procedures typically apply to all construction projects that disturb or occur within 17 stream channels and their associated riparian areas. 18
Limitations 19
Specific permit requirements such as a 401 certification or a U.S. Army Corps of 20 Engineers (COE) 404 permit, may be included in the contract documents. 21
Standards and Specifications 22
Planning 23
Proper planning, design, and construction techniques can minimize impacts normally 24 associated with in-stream construction activities. Poor planning can adversely affect 25 soil, fish, and wildlife resources, land uses, or land users. Planning should take the 26 following into account -- scheduling, avoiding in-stream construction, minimizing the 27 disturbance area and construction time period, using pre-disturbed areas, selecting a 28 crossing location, and selecting equipment. 29
Scheduling 30
Construction activities should be scheduled according to the relative sensitivity of the 31 environmental concerns and in accordance with scheduling. When working in or near 32 streams, work should be performed during the dry season. However, when closing 33 the site at the end of the project, wash any fines (see washing fines) that 34 accumulated in the channel back into the bed material to decrease pollution from the 35
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first rainstorm (“first flush”) of the season. When working near streams, erosion and 1 sediment controls (see silt fences, etc.) should be implemented to keep sediment out 2 of the stream channel. 3
Minimizing Disturbance 4
Minimize disturbance by selecting the narrowest crossing location, limiting the 5 number of equipment trips across a stream during construction, and minimizing the 6 number and size of work areas (equipment staging areas and spoil storage areas). 7 Place work areas at least 50 ft. from the stream channel. Provide stabilized access to 8 the stream when in-stream work is required. Field reconnaissance should be 9 conducted during the planning stage to identify work areas. 10
Using Pre-Disturbed Areas 11
Locate project sites and work areas in pre-disturbed areas when possible. 12
Selecting a Project Site 13
Avoid steep and unstable banks, highly erodible or saturated soils, or highly 14 fractured rock. 15
Select a project site that minimizes disturbance to aquatic species or habitat. 16
Selecting Equipment 17
Select equipment that reduces the amount of pressure exerted on the ground surface 18 and, therefore, reduces erosion potential, and/or use overhead or aerial access for 19 transporting equipment across drainage channels. Use equipment that exerts ground 20 pressures of less than 5 pounds or 6 pounds per sq. in., where possible. Low ground 21 pressure equipment includes wide or high flotation tires 34 in. to 72 in. wide, dual tires, 22 bogie axle systems, tracked machines, lightweight equipment, and central tire inflation 23 systems. 24
Preservation of Existing Vegetation 25
Preserve existing vegetation to the maximum extent possible. The preservation of 26 existing vegetation provides the following benefits: 27
Water Quality Protection 28
Vegetated buffers on slopes trap sediment and promote groundwater recharge. The 29 buffer width needed to maintain water quality ranges from 16 ft. to 98 ft. On gradual 30 slopes, most of the filtering occurs within the first 33 ft. Steeper slopes require a 31 greater width of vegetative buffer to provide water quality benefits. 32
As a general rule, the width of a buffer strip between a road and the stream is 33 recommended to be 48 ft. plus four times the percent slope of the land, measured 34 between the road and the top of streambank. 35
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Streambank Stabilization 1
The root system of riparian vegetation stabilizes streambanks by increasing tensile 2 strength in the soil. The presence of vegetation modifies the moisture condition of 3 slopes (infiltration, evapotranspiration, interception) and increases bank stability. 4
Riparian Habitat 5
Buffers of diverse riparian vegetation provide food and shelter for riparian and 6 aquatic organisms. Minimizing impacts to fisheries habitat is a major concern when 7 working near streams and rivers. Riparian vegetation provides shade, shelter, 8 organic matter (leaf detritus and large woody debris), and other nutrients that are 9 necessary for fish and other aquatic organisms. 10
When working near watercourses, it is important to understand the work site’s 11 placement in the watershed. Riparian vegetation in the headwater streams has a 12 greater impact on overall water quality than vegetation in downstream reaches. 13 Preserving existing vegetation upstream is necessary to maintain water quality, 14 minimize bank failure, and maximize riparian habitat downstream of the work site. 15
Vegetative Stabilization of Disturbed Soil Areas 16
Disturbed soil areas above the ordinary high water level should be immediately 17 stabilized by using the seeding, mulching, and/or soil binder BMPs listed above. Do not 18 place mulch, fertilizer, seed, and binders below the ordinary water level, as these 19 materials could wash into the channel and impact water quality. 20
Geotextiles, Plastic Covers, and Erosion Control Blankets/Mats 21
Install geotextiles, erosion control blankets, and plastic BMPs as described earlier to 22 stabilize disturbed channels and streambanks. Geotextile fabrics that are not 23 biodegradable are not appropriate for in-stream use. Additionally, geotextile fabric or 24 blankets placed in channels must be adequate to sustain anticipated hydraulic forces. 25
Earth Dikes/Drainage Swales, and Lined Ditches 26
Convey, intercept, or divert runoff from disturbed streambanks using earth dikes, 27 drainage swales, and lined ditches, as described above. 28
Limitations 29
Do not place earth dikes in watercourses, as these structures are only suited for 30 intercepting sheet flow and should not be used to intercept concentrated flow. 31
Place appropriately-sized outlet protection and energy dissipation devices at outlets 32 of pipes, drains, culverts, slope drains, diversion ditches, swales, conduits, or 33 channels. 34
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Slope Drains 1
Use slope drains to intercept and direct surface runoff or groundwater from dikes, 2 swales, and lined ditches that intercept flow away from the streambanks, discharging 3 into the lower stream channel with a stable outlet. 4
Limitations 5
Appropriately-sized outlet protection/velocity dissipation devices must be placed at 6 outlets to minimize erosion and scour. 7
7.2 TEMPORARY SEDIMENT CONTROL PRACTICES 8
Temporary sediment control practices include those practices that intercept and slow or 9 detain the flow of stormwater to allow sediment to settle and be trapped. These practices 10 can consist of installing temporary linear sediment barriers (such as silt fences and 11 sandbag barriers); providing fiber rolls, gravel bag berms, or check dams to break up 12 slope length or flow; or constructing a temporary sediment/de-silting basin on sediment 13 trap. Linear sediment barriers are typically placed below the toe of exposed and erodible 14 slopes, down slope of exposed soil areas, around temporary stockpiles, and at other 15 appropriate locations along the site perimeter. 16
Temporary sediment control practices include the BMPs listed in Table 7-6. 17
18
Temporary Sediment Control BMPs
Silt Fence
Sediment Basin
Sediment Trap
Check Dam
Fiber Rolls
Gravel Bag Berm
Street Sweeping
Sandbag Barrier
Storm Drain Inlet Protection
Streambank Sediment Control
Table 7-6: Temporary Sediment Control BMPs 19
7.2.1 SILT FENCE 20
Definition and Purpose 21
A silt fence is a temporary linear sediment barrier of permeable fabric designed to 22 intercept and slow the flow of sediment-laden sheet flow runoff. Silt fences allow 23 sediment to settle from runoff before water leaves the construction site. 24
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Appropriate Applications 1
Below the toe of exposed and erodible slopes 2
Down slope of exposed soil areas 3
Around temporary stockpiles 4
Along streams and channels 5
Along the perimeter of a project 6
Limitations 7
Not intended for use as mid-slope protection on slopes greater than 1:4 (V:H). 8
Must be maintained. 9
Must be removed and disposed of. 10
Not intended for use below slopes subject to creep, slumping, or landslides. 11
Not intended for use in streams, channels, drain inlets, or anywhere flow is 12 concentrated. 13
Not intended to divert flow. 14
Standards and Specifications 15
Design and Layout 16
The maximum length of slope draining to any point along the silt fence should 17 be 200 ft. or less. 18
Slope of area draining to silt fence should be less than 1:1 (V:H). 19
Limit to locations suitable for temporary ponding or deposition of sediment. 20
Fabric life span generally limited to between five and eight months. Longer 21 periods may require fabric replacement. 22
Silt fences should not be used in concentrated flow areas. 23
For slopes steeper than 1:2 (V:H) and that contain a high number of rocks or 24 large dirt clods that tend to dislodge, it may be necessary to install additional 25 protection immediately adjacent to the bottom of the slope prior to installing 26 the silt fence. Additional protection may be a chain link fence or a cable 27 fence. 28
For slopes adjacent to water bodies or other environmentally-sensitive areas, 29 additional temporary soil stabilization BMPs should be used. 30
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Materials 1
Silt fence fabric should be woven polypropylene with a minimum width of 2 36 in. and a minimum tensile strength of 0.45-kN. The fabric should conform 3 to the requirements in ASTM Designation D4632 and have an integral 4 reinforcement layer. The reinforcement layer should be a polypropylene, or 5 equivalent, net provided by the manufacturer. The permeability of the fabric 6 should be in conformance with the requirements in ASTM Designation 7 D4491. The contractor must submit a certificate of compliance. 8
Wood stakes should be commercial-quality lumber of the size and shape 9 shown on the plans. Each stake should be free from decay, splits, or cracks 10 longer than the thickness of the stake or other defects that would weaken the 11 stakes and cause the stakes to be structurally unsuitable. 12
Bar reinforcement may be used, and its size should be equal to a number 13 four or greater. End protection should be provided for any exposed bar 14 reinforcement. 15
Staples used to fasten the fence fabric to the stakes should be not less than 16 1.75 in. long and fabricated from 0.06-in or heavier wire. The wire used to 17 fasten the tops of the stakes together when joining two sections of fence 18 should be 0.12-in. or heavier wire. 19
Galvanizing of the fastening wire is not required. 20
Installation 21
Generally, silt fences should be used in conjunction with soil stabilization 22 source controls up-slope to provide effective erosion and sediment control. 23
The bottom of the silt fence should be keyed to a minimum of 12 in. 24
Trenches should not be excavated wider and deeper than necessary for 25 proper installation of the temporary linear sediment barriers. 26
Excavation of the trenches should be performed immediately before 27 installation of the temporary linear sediment barriers. 28
Construct silt fences with a setback of at least 3 ft. from the toe of a slope. 29 Where a silt fence is determined not to be practical due to specific site 30 conditions, the silt fence may be constructed at the toe of the slope, but as far 31 from the toe of the slope as practical. 32
Construct the length of each reach so that the change in base elevation along 33 the reach does not exceed one-third the height of the barrier; in no case 34 should the reach exceed 490 ft. 35
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Cross barriers should be a minimum of one-third and a maximum of half the 1 height of the linear barrier. 2
Maintenance and Inspection 3
Repair undercut silt fences. 4
Repair or replace split, torn, slumping, or weathered fabric. 5
Inspect silt fence weekly and after each significant rainfall. Immediately perform 6 any necessary maintenance and repairs. 7
Maintain silt fences to provide an adequate sediment holding capacity. Sediment 8 should be removed when the sediment accumulation reaches one-third of the 9 barrier height. Removed sediment should be incorporated in the project or 10 disposed of outside of the right-of-way in conformance with the project 11 requirements. 12
Silt fences that are damaged and become unsuitable for their intended purpose 13 should be removed from the site of work, disposed of outside of the highway 14 right-of-way in conformance with the contract requirements, and replaced with 15 new silt fence barriers. 16
Holes, depressions, or other ground disturbance caused by the removal of the 17 temporary silt fences should be backfilled and repaired. 18
Remove silt fence when no longer needed. Fill and compact post holes and 19 anchorage trench, remove sediment accumulation, and grade the fence 20 alignment to blend with the adjacent ground. 21
7.2.2 SEDIMENT BASIN 22
Definition and Purpose 23
A sediment basin (pond) is a temporary basin formed by excavating and/or constructing 24 an embankment so that sediment-laden runoff is temporarily detained under quiescent 25 conditions, allowing sediment to settle out before the runoff is discharged. 26
Appropriate Applications 27
Prior to leaving a construction site, stormwater runoff must pass through a sediment 28 basin or other appropriate sediment removal BMP. 29
A sediment basin should be used where the contributing drainage area is 3 acres or 30 more. Ponds must be used in conjunction with erosion control practices to reduce the 31 amount of sediment flowing into the basin. 32
33
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Limitations 1
Alternative BMPs must be thoroughly investigated for erosion control before selecting 2 temporary sediment basins. 3
Sediment basins require large surface areas to permit the settling of sediment. 4
Sediment basins are not appropriate for drainage areas greater than 75 acres. 5
Sediment basins should not be located in live streams. 6
For safety reasons, basins should have protective fencing. 7
The size of sediment basins may be limited by the availability of right-of-way. 8
Standards and Specifications 9
Try to limit the contributing area to the sediment basin to only the runoff from the 10 disturbed soil areas. Use temporary concentrated flow conveyance controls to 11 divert runoff from undisturbed areas. This will help to keep the sediment basin as 12 small as possible. 13
Sediment Basin 14
At a minimum, sediment basins should be designed as follows: 15
If permanent runoff control facilities are part of the project, they should be used 16 for sediment retention. The surface area requirements of the sediment basin 17 must be met. This may require enlarging the permanent basin to comply with the 18 surface area requirements. If a permanent control structure is used, it must have 19 the outlet controls blocked to accommodate the sedimentation design. 20
Outlets for sediment basins should be an overflow structure, weir, berm, or 21 floating intake so that discharge is skimmed off of the upper pond surface. 22
The use of infiltration facilities for sedimentation basins during construction tends 23 to clog the soils and reduce their capacity to infiltrate. If infiltration facilities are to 24 be used, the sides and bottom of the facility must only be roughly excavated to a 25 minimum of 2 ft. above the final grade. Final grading of the infiltration facility 26 should occur only when all contributing drainage areas are fully stabilized. The 27 infiltration pre-treatment facility should be fully constructed and used with the 28 sedimentation basin to help prevent clogging. 29
Sediment basins should be designed for the three-year frequency storm 30
Determining Pond Geometry 31
o Obtain the discharge from the hydrologic calculations of the peak flow for 32 the three-year runoff event (Q2). The 10-year peak flow should be used if 33 the project size, expected timing and duration of construction, or 34
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downstream conditions warrant a higher level of protection. If no 1 hydrologic analysis is required, the Rational Method may be used. 2
o Determine the required surface area at the top of the riser pipe with the 3 equation: 4
SA = 2 x Q2/0.00096, or 2,080 sq. ft. per cfs of inflow. 5
(This equation, along with the minimum depth of pond, will provide 6 the necessary volume for a time of residence to allow a 0.02 mm 7 (medium silt) particle with an assumed density of 2.65 g/cm3, 8 which has a settling velocity of 0.00096 ft/sec with a safety factor 9 of two, to settle out). 10
The basic geometry of the pond can now be determined using the following 11 design criteria: 12
o Required Surface Area (SA), from Step 2, above, at top of riser 13
o Minimum 3.5-ft. depth from top of riser to bottom of pond 14
o Maximum 3:1 interior side slopes and maximum 2:1 exterior slopes 15
The interior slopes can be increased to a maximum of 2:1 if fencing is provided at or 16 above the maximum water surface. 17
One ft. of freeboard between the top of the overflow device and the top of the 18 pond. 19
Flat bottom. 20
The overflow device should be capable of handling the 10-year sized storm peak 21 flow without overtopping the top of pond. 22
Length-to-width ratio between 3:1 and 6:1. 23
Typical overflow outlets can be a rock riprapped weir surface discharging to an 24 erosion protected channel or other conveyance system; a vertical pipe or pre-25 cast structure riser connected to a discharge pipe also discharging to an erosion-26 protected outlet or other conveyance system; a pumped discharge where the 27 pump intake is located just under the pond surface using anti-vortex baffles; and 28 a floating overflow weir connected to a flexible pipe that allows water to skim 29 immediately from the surface of the water. A floating overflow weir/pipe is usually 30 associated with either a pumped discharge using automatic floats to control the 31 pond surface levels or another permanent control structure to maintain the 32 maximum design water surface. 33
34
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Maintenance Standards 1
Sediment should be removed from the pond when it reaches 1 ft. in depth. 2
Inspect sediment basins before and after rainfall events and weekly during the 3 rest of the rainy season. During extended rainfall events, inspect at least every 4 24 hours. 5
Examine basin banks for seepage and structural soundness. 6
Check inlet and outlet structures and spillway for any damage or obstructions. 7
Repair damage and remove obstructions as needed. 8
7.2.3 SEDIMENT TRAP 9
Definition and Purpose 10
A sediment trap is a temporary containment area that allows sediment in collected 11 stormwater to settle out during infiltration or before the runoff is discharged through a 12 stabilized spillway. Sediment traps are formed by excavating or constructing an earthen 13 embankment across a waterway or low drainage area. 14
Appropriate Applications 15
Sediment traps may be used on construction projects where the drainage area is 16 less than 3 acres. Traps should be placed prior to where sediment-laden 17 stormwater enters a storm drain or watercourse. 18
This BMP may be implemented on a project-by-project basis with other BMPs, 19 when necessary and feasible. 20
As a supplemental control, sediment traps provide additional protection for a 21 water body or for reducing sediment before it enters a drainage system. 22
Limitations 23
Sediment traps require large surface areas to permit infiltration and the settling of 24 sediment. 25
Sediment traps are not appropriate for drainage areas greater than 3 acres. 26
Sediment traps only remove large and medium-sized particles and require 27 upstream erosion control. 28
Sediment traps are attractive and dangerous to children, requiring protective 29 fencing. 30
Sediment traps should not be located in live streams. 31
The size of sediment traps may be limited by the availability of right-of-way. 32
33
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Standards and Specifications 1
Construct sediment traps prior to the rainy season and construction activities. 2
Traps should be situated according to the following criteria: 3
o By excavating a suitable area or where a low embankment can be 4 constructed across a swale. 5
o Should be designed for the three-year storm frequency. 6
o Where failure would not cause loss of life or property damage. 7
o To provide access for maintenance, including sediment removal and 8 sediment stockpiling, in a protected area. 9
To determine the sediment trap geometry, first calculate the design SA of the 10 trap, measured at the invert of the weir. Use the following equation: 11
SA = 2 x Q2/0.00096, or 2,080 sq. ft. per cfs of inflow 12
Where: 13
Q2 = Design inflow based on the peak discharge from the 14 developed three-year runoff event from the contributing drainage 15 area as computed in the hydrologic analysis. 16
o To aid in determining sediment depth, all sediment traps should have a 17 staff gauge with a prominent mark 1 ft. above the bottom of the trap. 18
o Sediment traps should have a stabilized outlet by skimming the surface 19 water. This is usually performed with an overflow weir, such as a pipe or 20 structural riser discharging by connecting pipe to another conveyance 21 system or erosion protected outlet to a waterway; by a riprapped overflow 22 weir; by pumped discharge where the intake is located just below the trap 23 surface with an anti-vortex baffle; or using a floating skimmer with a 24 flexible pipe. 25
Sediment traps may not be feasible on utility projects due to the limited work 26 space or the short-term nature of the work. Portable tanks may be used in place 27 of sediment traps for utility projects. 28
Maintenance and Inspection 29
Inspect sediment traps before and after rainfall events and weekly during the rest 30 of the rainy season. Remove accumulated sediment when the volume has 31 reached one-third the original trap volume. 32
Inspect the outlet structure for any damage or obstructions. Repair damage and 33 remove obstructions as needed or as directed by the Resident Engineer (RE). 34
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7.2.4 CHECK DAMS 1
Definition and Purpose 2
Check dams reduce scour and channel erosion by reducing flow velocity and 3 encouraging sediment settlement. A check dam is a small device constructed of rock, 4 gravel bags, sandbags, fiber rolls, or other proprietary product placed across a natural or 5 man-made channel or drainage ditch. 6
Appropriate Applications 7
Check dams may be installed: 8
In small open channels that drain 10 acres or less. 9
In steep channels where stormwater runoff velocities exceed 4.9 ft. per second. 10
During the establishment of grass linings in drainage ditches or channels. 11
In temporary ditches where the short length of service does not warrant the 12 establishment of erosion-resistant linings. 13
This BMP may be implemented on a project-by-project basis with other BMPs 14 when determined necessary and feasible. 15
Limitations 16
Check dams should not be used in live streams. 17
Check dams are not appropriate in channels that drain areas greater than 10 18 acres. 19
Check dams should not be placed in channels that are already grass-lined, 20 unless erosion is expected, as installation may damage vegetation. 21
Check dams require extensive maintenance following high-velocity flows. 22
Check dams promote sediment trapping, which can be re-suspended during 23 subsequent storms or removal of the check dam. 24
Check dams should not be constructed from straw bales or silt fence. 25
Standards and Specifications 26
Check dams should be placed at a distance and height to allow small pools to 27 form behind them. Install the first check dam approximately 16 ft. from the outfall 28 device and at regular intervals based on slope gradient and soil type. 29
For multiple check dam installation, backwater from downstream check dams 30 should reach the toe of the upstream dam. 31
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High flows (typically, a two-year storm or larger) should safely flow over the 1 check dam without an increase in upstream flooding or damage to the check 2 dam. 3
Where grass is used to line ditches, check dams should be removed when grass 4 has matured sufficiently to protect the ditch or swale. 5
Rock should be placed individually by hand or by mechanical methods (no 6 dumping of rock) to achieve complete ditch or swale coverage. 7
Fiber rolls may be used as check dams. 8
Gravel bags may be used as check dams with the following specifications: 9
o Bag Material: Bags should be either polypropylene, polyethylene, or 10 polyamide woven fabric (minimum unit weight four ounces per square 11 yard), with a mullen burst strength exceeding 300 pounds per sq. in., in 12 conformance with the requirements in ASTM Designation D3786, and a 13 UV stability exceeding 70 percent, in conformance with the requirements 14 in ASTM Designation D4355. 15
o Bag Size: Each gravel-filled bag should have a length of 18 in., a width of 16 12 in., a thickness of 3 in., and a mass of approximately 33 pounds. Bag 17 dimensions are nominal and may vary based on locally-available 18 materials. Alternative bag sizes should be submitted to the RE for 19 approval prior to deployment. 20
o Fill Material: Fill material should be between 0.4 in. and 0.8 in. in 21 diameter, and should be clean and free from clay balls, organic matter, 22 and other deleterious materials. The opening of gravel-filled bags should 23 be secured so that gravel does not escape. Gravel-filled bags should be 24 between 28 pounds and 48 pounds in mass. 25
Installation 26
Install check dams along a level contour. 27
Tightly abut bags and/or rock and stack using a pyramid approach. Dams 28 should not exceed about 3.5 ft. in height. 29
The upper rows of gravel bags or rock should overlap joints in lower rows. 30
Maintenance and Inspection 31
Inspect check dams after each significant rainfall event. Repair damage as 32 needed. 33
Remove sediment when the depth reaches one-third of the check dam 34 height. 35
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Remove accumulated sediment prior to permanent seeding or soil 1 stabilization. 2
Remove check dam and accumulated sediment when check dams are no 3 longer needed. 4
Removed sediment should be incorporated into the project or disposed of 5 outside of the highway right-of-way in conformance with the contract 6 requirements. 7
7.2.5 FIBER ROLLS/WATTLES 8
Definition and Purpose 9
A fiber roll consists of wood excelsior, rice or wheat straw, or coconut fibers rolled or 10 bound into a tight tubular roll and placed on the toe and face of slopes to intercept runoff, 11 reduce its flow velocity, release the runoff as sheet flow, and provide removal of 12 sediment from the runoff. Fiber rolls may also be used for inlet protection and as check 13 dams under certain situations. 14
Appropriate Applications 15
Use fiber rolls along the toe, top, face, and at-grade breaks of exposed and 16 erodible slopes to shorten slope length and spread runoff as sheet flow. 17
Use fiber rolls below the toe of exposed and erodible slopes. 18
Fiber rolls may be used as check dams in unlined ditches if approved by the RE. 19
Fiber rolls may be used for drain inlet protection if approved by the RE. 20
Fiber rolls may be used down slope of exposed soil areas. 21
Fiber rolls may be used around temporary stockpiles. 22
Fiber rolls may be used along the perimeter of a project. 23
Limitations 24
Runoff and erosion may occur if a fiber roll is not adequately trenched in. 25
Fiber rolls at the toe of slopes steeper than 1V:5H may require the use of 20-in. 26 diameter fiber rolls or installations achieving the same protection (i.e., stacked 27 fiber rolls of a smaller diameter). 28
Fiber rolls are difficult to move once saturated. 29
Fiber rolls could be transported by high flows if not properly staked and trenched 30 in. 31
Fiber rolls have a limited sediment capture zone. 32
Do not use fiber rolls on slopes subject to creep, slumping, or landslide. 33
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Standards and Specifications 1
Assembly of Field-Rolled Fiber Roll 2
Roll the length of an erosion control blanket into a tube a minimum of 8 in. in 3 diameter. 4
Bind the roll at each end and every 4 ft. along the length of the roll with jute-5 type twine. 6
Installation 7
Slope inclination of 1:4 or flatter: Fiber rolls should be placed on slopes 20 ft. 8 apart. 9
Slope inclination of 1:4 to 1:2: Fiber rolls should be placed on slopes 15 ft. 10 apart. 11
Slope inclination 1:2 or greater: Fiber rolls should be placed on slopes 10 ft. 12 apart. 13
Stake fiber rolls into a 2-in. to 4-in. deep trench. 14
Drive stakes at the end of each fiber roll, and space them 2 ft. to 4 ft. apart. 15
Use wood stakes with a nominal classification of 3/4 in. by 3/4 in. and a 16 minimum length of 24 in. 17
If more than one fiber roll is placed in a row, the rolls should be overlapped, 18 not abutted. 19
Removal 20
Fiber rolls are typically left in place. 21
If fiber rolls are removed, collect and dispose of sediment accumulation, and 22 fill and compact holes, trenches, depressions, or any other ground 23 disturbance to blend with the adjacent ground. 24
Maintenance and Inspection 25
Repair or replace split, torn, unraveling, or slumping fiber rolls. 26
Inspect fiber rolls weekly and immediately after a significant rain. Perform 27 maintenance as needed. 28
Maintain fiber rolls to provide an adequate sediment-holding capacity. Sediment 29 should be removed when the sediment accumulation reaches three-quarters of 30 the barrier height. Removed sediment should be incorporated into the project or 31 disposed of outside of the project limits in conformance with the contract 32 requirements. 33
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7.2.5.1 GRAVEL BAG BERM 1
Definition and Purpose 2
A gravel bag berm consists of a single row of gravel bags that are installed end-to-end to 3 form a barrier across a slope to intercept runoff, reduce its flow velocity, release the 4 runoff as sheet flow, and provide some sediment removal. Gravel bags can be used 5 where flows are moderately concentrated such as ditches, swales, and storm drain 6 inlets, to divert and/or detain flows. 7
Appropriate Applications 8
This BMP is used for the same purposes as fiber rolls and wattles. 9
Limitations 10
Degraded gravel bags may rupture when removed, spilling contents. 11
Installation can be labor intensive. 12
Gravel bags have limited durability for long-term projects. 13
When used to detain concentrated flows, maintenance requirements increase. 14
Standards and Specifications 15
Materials 16
See the material specifications of gravel bags used for check dams. 17
Installation 18
When used as a linear control for sediment removal: 19
o Install along a level contour. 20
o Turn the ends of the gravel bag row upslope to prevent flow around 21 the ends. 22
o Generally, gravel bag barriers should be used in conjunction with 23 temporary soil stabilization controls up slope to provide effective 24 erosion and sediment control. 25
When used for concentrated flows: 26
o Stack gravel bags to their required height using a pyramid approach. 27
o The upper rows of gravel bags should overlap joints in lower rows. 28
Construct gravel bag barriers with a setback at least 3 ft. from the toe of a 29 slope. Where it is determined to not be practicable due to specific site 30 conditions, the gravel bag barrier may be constructed at the toe of the slope, 31 but as far from the toe of the slope as practicable. 32
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Maintenance and Inspection 1
Inspect gravel bag berms weekly throughout the rainy season. 2
Reshape or replace gravel bags as needed. 3
Repair washouts or other damages as needed. 4
Inspect gravel bag berms for sediment accumulations, and remove sediments 5 when accumulation reaches one-third of the berm height. Removed sediment 6 should be incorporated into the project or disposed of outside of the project limits 7 in conformance with the contract requirements. 8
Remove gravel bag berms when no longer needed. Remove sediment 9 accumulations and clean, re-grade, and stabilize the area. 10
7.2.6 STREET SWEEPING AND VACUUMING 11
Definition and Purpose 12
Street sweeping and vacuuming are practices to remove tracked sediment to prevent the 13 sediment from entering a storm drain or watercourse. 14
Appropriate Applications 15
These practices are implemented anywhere sediment is tracked from the project site 16 onto public or private paved roads, typically at points of ingress or egress. 17
Limitations 18
Sweeping and vacuuming may not be effective when soil is wet or muddy. 19
Standards and Specifications 20
Kick brooms or sweeper attachments should not be used. 21
Inspect potential sediment tracking locations daily. 22
Visible sediment tracking should be swept and/or vacuumed daily. 23
If not mixed with debris or trash, consider incorporating the removed sediment 24 back into the project. 25
Maintenance and Inspection 26
Inspect ingress/egress access points daily and sweep tracked sediment as 27 needed. 28
Be careful not to sweep up any unknown substance or any object that may be 29 potentially hazardous. 30
Adjust brooms frequently; maximize the efficiency of sweeping operations. 31
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After sweeping is finished, properly dispose of sweeper wastes at an approved 1 disposal site in conformance with the contract requirements. 2
7.2.6.1 SANDBAG BARRIER 3
Definition and Purpose 4
A sandbag barrier is a temporary linear sediment barrier consisting of stacked sandbags 5 designed to intercept and slow the flow of sediment-laden sheet flow runoff. Sandbag 6 barriers allow sediment to settle from runoff before water leaves the construction site. 7
Appropriate Applications 8
This BMP has the same application as the gravel bag berms. 9
Standards and Specifications 10
Materials 11
Sandbag Material: Sandbags should be woven polypropylene, polyethylene, 12 or polyamide fabric (minimum unit weight four ounces per square yard), with 13 a mullen burst strength exceeding 300 pounds per sq. in., in conformance 14 with the requirements in ASTM Designation D3786 and with a UV stability 15 exceeding 70 percent, in conformance with the requirements in ASTM 16 Designation D4355. The use of burlap is not acceptable. 17
Sandbag Size: Each sand-filled bag should have a length of 18 in., a width of 18 12 in., a thickness of 3 in., and a mass of approximately 33 pounds. Bag 19 dimensions are nominal and may vary based on locally-available materials. 20
Fill Material: All sandbag fill material should be non-cohesive, permeable 21 material free from clay and deleterious material, conforming to the provisions 22 in Standard Specification 703, “Permeable Backfill.” The requirements for the 23 durability index and sand equivalent do not apply. 24
Installation and Maintenance 25
The same installation and maintenance requirements apply as noted for gravel bag 26 berms. 27
7.2.7 STORM DRAIN INLET PROTECTION 28
Definition and Purpose 29
Storm drain inlet protection is achieved with devices used at storm drain inlets that are 30 subject to runoff from construction activities to detain and/or to filter sediment-laden 31 runoff, to allow sediment to settle and/or to filter sediment prior to discharge into storm 32 drainage systems or watercourses. 33
34
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Appropriate Applications 1
Where ponding will not encroach into highway traffic. 2
Where sediment-laden surface runoff may enter an inlet. 3
Where disturbed drainage areas have not yet been permanently stabilized. 4
Where the drainage area is 1 acre or less. 5
Limitations 6
Storm drain inlet protection requires an adequate area for water to pond without 7 encroaching upon the traveled way and should not present an obstacle to 8 oncoming traffic. 9
Storm drain inlet protection may be best if other methods of temporary protection 10 are used in conjunction with it. 11
Sediment removal may be difficult in high flow conditions or if runoff is heavily 12 sediment laden. If high flow conditions are expected, use other on-site sediment- 13 trapping techniques, such as check dams, in conjunction with inlet protection. 14
Frequent maintenance is required. 15
Filter fabric fence inlet protection is appropriate in open areas that are subject to 16 sheet flow and for flows not exceeding 0.5 cu. ft. per second. 17
Gravel bag barriers for inlet protection are applicable when sheet flows or 18 concentrated flows exceed 0.5 cu. ft. per second, and it is necessary to allow for 19 overtopping to prevent flooding. 20
Fiber rolls and foam barriers are not appropriate for locations where they cannot 21 be properly anchored to the surface. 22
Excavated drop inlet sediment traps are appropriate where relatively heavy flows 23 are expected and overflow capability is needed. 24
Standards and Specifications 25
Identify existing and/or planned storm drain inlets that have the potential to receive 26 sediment-laden surface runoff. Determine if storm drain inlet protection is needed and 27 which method to use. 28
Methods and Installation 29
Drainage Inlet Protection Type 1 – Filter Fabric Fence. The filter fabric fence 30 protection is similar to constructing a silt fence. Do not place filter fabric 31 underneath the inlet grate because the collected sediment may fall into the drain 32 inlet when the fabric is removed or replaced. 33
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Drainage Inlet Protection Type 2 – Excavated Drop Inlet Sediment Trap. The 1 excavated drop inlet sediment trap is similar to constructing a temporary silt trap 2 around the inlet structure. Use the inlet grate as the overflow discharge control 3 weir for the silt trap. Size the excavated trap to provide a minimum storage 4 capacity calculated at the rate of 67 cu. yd per acre of drainage area. 5
Drainage Inlet Protection Type 3 – Gravel Bag. Flow from a severe storm 6 should not overtop the berm. In areas of high clay and silts, use filter fabric and 7 gravel as additional filter media. Construct gravel bags as level berms upstream 8 of the inlet structure. Gravel or sand bags should be used due to their high 9 permeability. 10
Drainage Inlet Protection Type 4 – Foam Barriers and Fiber Rolls. Foam 11 barriers or fiber rolls are placed around the inlet and keyed and anchored to the 12 surface. Foam barriers and fiber rolls are intended for use as inlet protection 13 where the area around the inlet is unpaved and the foam barrier or fiber roll can 14 be secured to the surface. 15
Maintenance and Inspection 16
General 17
Inspect all inlet protection devices before and after every rainfall event and 18 weekly during the rest of the rainy season. During extended rainfall events, 19 inspect inlet protection devices at least once every 24 hours. 20
Inspect the storm drain inlet after severe storms in the rainy season to check 21 for bypassed material. 22
Remove all inlet protection devices after the site is stabilized, or when the 23 inlet protection is no longer needed. 24
o Bring the disturbed area to final grade and smooth and compact it. 25 Appropriately stabilize all bare areas around the inlet. 26
o Clean and re-grade the area around the inlet and clean the inside of 27 the storm drain inlet, as it must be free of sediment and debris at the 28 time of final inspection. 29
Requirements by Method 30
Type 1 – Filter Fabric Fence 31
This method should be used for drain inlets requiring protection in areas 32 where finished grade is established and erosion control seeding has been 33 applied or is pending. 34
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Make sure the stakes are securely driven in the ground and are structurally 1 sound (i.e., not bent, cracked, or splintered) and are reasonably 2 perpendicular to the ground. Replace damaged stakes. 3
Replace or clean the fabric when the fabric becomes clogged with sediment. 4 Make sure the fabric does not have any holes or tears. Repair or replace 5 fabric as needed or as directed by the RE. 6
At a minimum, remove the sediment behind the fabric fence when 7 accumulation reaches one-third the height of the fence or barrier height. 8 Removed sediment should be incorporated into the project at locations 9 designated by the RE or disposed of outside of the highway right-of-way, in 10 conformance with the Standard Specifications. 11
Type 2 – Excavated Drop Inlet Sediment Trap 12
This method may be used for drain inlets requiring protection in areas that 13 have been cleared and grubbed, and where exposed soil areas are subject to 14 grading. 15
Remove sediment from the trap when the volume of the basin has been 16 reduced by one-half. 17
Type 3 – Gravel Bag Barrier 18
This method may be used for drain inlets surrounded by asphalt concrete or 19 paved surfaces. 20
Inspect bags for holes, gashes, and snags. 21
Check gravel bags for proper arrangement and displacement. Remove the 22 sediment behind the barrier when it reaches one-third the height of the 23 barrier. 24
Type 4 – Foam Barriers and Fiber Rolls 25
This method may be used for drain inlets requiring protection in areas that 26 have been cleared and grubbed, and where exposed soil areas are subject to 27 grading. 28
Check the foam barrier or fiber roll for proper arrangement and displacement. 29 Remove the sediment behind the barrier when it reaches one-third the height 30 of the barrier. Removed sediment should be incorporated into the project at 31 locations designated by the RE or disposed of outside of the highway right-of-32 way in conformance with the Standard Specifications. 33
7.2.8 STREAMBANK SEDIMENT CONTROL 34
These BMPs are used to isolate work on the streambank from the stream channel. 35
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Silt Fences 1
Install silt fences to control sediment movement from the working side of the 2 stream bank and the channel bottom. 3
If placed within the water, the silt fence should be protected from high velocity 4 flow. The water surface on each side of the silt fence must be level creating a 5 pond on the working side, thus allowing the sediment to settle out. 6
Fiber Rolls 7
Install fiber rolls along the slope contour above the high-water level to intercept 8 runoff, reduce flow velocity, release the runoff as sheet flow, and provide removal 9 of sediment from the runoff. In a stream environment, fiber rolls should be used 10 in conjunction with other sediment control methods, such as a silt fence or gravel 11 bag berm. Install the silt fence, gravel berm, or other erosion control methods 12 along the toe of the slope above the ordinary water level. 13
Gravel Bag Berm 14
A gravel bag berm can be used to intercept and slow the flow of sediment-laden sheet 15 flow runoff. In a stream environment, gravel bag berms can allow sediment to settle from 16 runoff before water leaves the construction site, and can be used to isolate the work 17 area from the stream. 18
Limitations 19
Gravel bag barriers are not recommended as a perimeter sediment control practice 20 around streams. 21
Rock Filters 22
Description and Purpose 23
Rock filters are temporary erosion control barriers composed of rock that is anchored 24 in place using woven wire sheathing (gabions). Rock filters detain the sediment-25 laden runoff, retain the sediment, and release the water as sheet flow at a reduced 26 velocity. 27
Applications 28
Apply rock filters near the toe of slopes that may be subject to flow and rill erosion. 29
Limitations 30
Inappropriate for drainage areas greater than 5 acres. 31
Requires sufficient space for ponded water. 32
Ineffective for diverting runoff because filters allow water to slowly seep 33 through. 34
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Berms difficult to remove when construction is complete. 1
Unsuitable in developed areas or locations where aesthetics is a concern. 2
Specifications 3
Rock: Open-graded rock, 0.75 in to 5 in. for concentrated flow applications. 4
Rock contained in gabions consisting of woven wire sheathing: 1 in. in 5 diameter, hexagonal mesh, galvanized 20-gauge. 6
In construction traffic areas, maximum rock berm heights should be 12 in. 7 Berms should be constructed every 300 ft. on slopes less than 5:100 (V:H) 8 (5 percent), every 200 ft. on slopes between 5:100 (V:H) (5 percent) and 9 10:100 (V:H) (10 percent), and every 100 ft. on slopes greater than 10:100 10 (V:H) (10 percent). 11
Maintenance and Inspection 12
Inspect berms before and after each significant rainfall event and weekly 13 throughout the rainy season. 14
Reshape berms as needed and replace lost or dislodged rock and/or filter 15 fabric. 16
Inspect for sediment accumulation, remove sediment when depth reaches 17 one-third of the berm height or 12 in., whichever occurs first. 18
K-Rail 19
Description and Purpose 20
This is temporary sediment control that uses K-rails (pre-cast concrete traffic 21 barriers) to form the sediment deposition area, or to isolate the near-bank 22 construction area. Install K-rails at the toe of the slope (see also the 23 description for the BMP “Clear Water Diversion.”) 24
Barriers are placed end-to-end in a pre-designed configuration, and gravel- 25 filled bags are used at the toe of the barrier and also at their abutting ends to 26 seal and prevent the movement of sediment beneath or through the barrier 27 walls. 28
Appropriate Applications 29
This technique is useful at the toe of embankments, and cut or fill slopes. 30
Limitations 31
The K-rail method is not watertight, and its proper use should be considered 32 accordingly. 33
34
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Inspection and Maintenance 1
Inspect BMPs daily during construction. 2
Maintain and repair BMPs as required. 3
Remove accumulated sediment. 4
7.3 WIND EROSION CONTROL PRACTICES 5
Wind erosion control consists of applying water and/or other dust palliatives as 6 necessary to prevent or alleviate dust nuisance. 7
Definition and Purpose 8
Wind erosion control consists of applying water and/or other dust palliatives as 9 necessary to prevent or alleviate erosion by the forces of wind. Covering small stockpiles 10 or areas is an alternative to applying water or other dust palliatives. 11
Appropriate Applications 12
This practice is implemented on all exposed soils subject to wind erosion. 13
Limitations 14
Its effectiveness depends on soil, temperature, humidity, and wind velocity. 15
Standards and Specifications 16
Water should be applied by means of pressure-type distributors or pipelines 17 equipped with a spray system or hoses and nozzles that will ensure even 18 distribution. 19
All distribution equipment should be equipped with a positive means of shutoff. 20
Unless water is applied by means of pipelines, at least one mobile unit should be 21 available at all times to apply water or dust palliative to the project. 22
Materials applied as temporary soil stabilizers and soil binders will also provide 23 wind erosion control benefits. 24
Maintenance and Inspection 25
Check areas that have been protected to ensure coverage. 26
7.4 TRACKING CONTROL BMPs 27
Tracking control consists of preventing or reducing vehicle tracking from entering a 28 storm drain or watercourse. Tracking control BMPs are shown in Table 7-7. 29
30
31 32
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Tracking Control BMPs
Stabilized Construction Entrance/Exit
Stabilized Construction Roadway
Entrance/Outlet Tire Wash
Table 7-7: Tracking Control BMPs 1
7.4.1 STABILIZED CONSTRUCTION ENTRANCE/EXIT 2
Definition and Purpose 3
A stabilized construction access is defined by a point of entrance/exit to a construction 4 site that is stabilized to reduce the tracking of mud and dirt onto public roads by 5 construction vehicles. 6
Appropriate Applications 7
Use tracking control at construction sites: 8
o Where dirt or mud can be tracked onto public roads 9
o Adjacent to water bodies 10
o Where poor soils are encountered 11
o Where dust is a problem during dry weather conditions 12
This BMP may be implemented on a project-by-project basis in addition to other BMPs. 13
Limitations 14
Site conditions will dictate the design and need. 15
Standards and Specifications 16
Limit the points of entrance/exit to the construction site. 17
Limit the speed of vehicles to control dust. 18
Properly grade each construction entrance/exit to prevent runoff from leaving the 19 construction site. 20
Route runoff from stabilized entrances/exits through a sediment-trapping device 21 before discharge. 22
Design stabilized entrances/exits to support the heaviest vehicles and equipment 23 that will use it. 24
Select construction access stabilization (aggregate, asphaltic concrete, concrete) 25 based on longevity, required performance, and site conditions. 26
The use of constructed/manufactured steel plates with ribs for entrance/exit 27 access is another alternative. 28
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If aggregate is selected, place crushed aggregate over geotextile fabric to at 1 least 12 in. deep. Crushed aggregate greater than 3 in. and smaller than 6 in. 2 should be used. 3
Designate combination or single-purpose entrances and exits to the construction 4 site. 5
Require all employees, subcontractors, and suppliers to use the stabilized 6 construction access. 7
All exit locations intended to be used continuously and for a period of time should 8 have stabilized construction entrance/exit BMPs. 9
Maintenance and Inspection 10
Inspect the area routinely for damage and assess the effectiveness of the BMP. 11 Remove aggregate, and separate and dispose of sediment if the construction 12 entrance/exit is clogged with sediment or as directed by the RE. 13
Keep all temporary roadway ditches clear. 14
Inspect for damage and repair as needed. 15
7.4.2 STABILIZED CONSTRUCTION ROADWAY 16
Definition and Purpose 17
A stabilized construction roadway is a temporary access road. It is designed for the 18 control of dust and erosion created by vehicular tracking. 19
Appropriate Applications 20
Where mud tracking is a problem during wet weather. 21
Where dust is a problem during dry weather. 22
Adjacent to water bodies. 23
Where poor soils are encountered. 24
Where there are steep grades and additional traction is needed. 25
Limitations 26
Materials will likely need to be removed prior to final project grading and 27 stabilization. 28
Site conditions will dictate design and need. 29
A stabilized construction roadway may not be applicable to projects of a very 30 short duration. 31
Limit the speed of vehicles to control dust. 32
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Standards and Specifications 1
Design stabilized access to support the heaviest vehicles and equipment that will 2 use it. 3
Stabilize roadway using aggregate, asphalt concrete, or concrete based on 4 longevity, required performance, and site conditions. 5
Coordinate materials with those used for stabilized construction entrance/exit 6 points. 7
If aggregate is selected, place crushed aggregate over geotextile fabric to at 8 least 12 in. deep, or place aggregate to a depth suitable for the proposed 9 loadings. Crushed aggregate greater than 75 mm (3 in.) and smaller than 6 in. 10 should be used. 11
Maintenance and Inspection 12
Inspect routinely for damage and repair as needed. 13
Keep all temporary roadway ditches clear. 14
7.4.3 ENTRANCE/OUTLET TIRE WASH 15
Definition and Purpose 16
A tire wash is an area located at stabilized construction access points to remove 17 sediment from tires and undercarriages, and to prevent sediment from being transported 18 onto public roadways. 19
Appropriate Applications 20
Tire washes may be used on construction sites where dirt and mud tracking onto 21 public roads by construction vehicles may occur. 22
Limitations 23
Tire washes require a supply of washwater. 24
Tire washes require a turnout or double-wide exit to avoid having entering 25 vehicles drive through the wash area. 26
Standards and Specifications 27
Incorporate with a stabilized construction entrance/exit. 28
Construct on level ground when possible, on a pad of coarse aggregate greater 29 than 3 in. and smaller than 6 in. A geotextile fabric should be placed below the 30 aggregate. 31
Wash rack should be designed and constructed/manufactured for anticipated 32 traffic loads. 33
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Provide a drainage ditch that will convey the runoff from the wash area to a 1 sediment-trapping device. The drainage ditch should be of sufficient grade, width, 2 and depth to carry the wash runoff. 3
Require all employees, subcontractors, and others who leave the site with mud-4 caked tires and/or undercarriages to use the wash facility. 5
Maintenance and Inspection 6
Remove accumulated sediment in the wash rack and/or sediment trap to 7 maintain system performance. 8
Inspect routinely for damage and repair as needed. 9
7.5 NON-STORMWATER MANAGEMENT PRACTICES 10
Non-stormwater management BMPs are source control BMPs that prevent pollution by 11 limiting or reducing potential pollutants at their source before they come into contact with 12 stormwater. These practices involve day-to-day operations of the construction site and 13 are usually under the contractor’s control. These BMPs are also referred to as good 14 housekeeping practices, which involve keeping a clean, orderly construction site. 15 Table 7-8 lists the non-stormwater management BMPs. 16
17
Non-Stormwater BMPs
Water Conservation Practices
De-watering Operations
Temporary Stream Crossing
Clean Water Diversion
Potable Water/Irrigation
Vehicle and Equipment Cleaning
Vehicle and Equipment Fueling
Vehicle and Equipment Maintenance
Pile-Driving Operations
Concrete Curing
Material and Equipment Use Over Water
Concrete Finishing
Structure Demolition Over or Adjacent to Water
Table 7-8: Non-Stormwater BMPs 18
19 20 21
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7.5.1 WATER CONSERVATION PRACTICES 1
Definition and Purpose 2
Water conservation practices are activities that use water during the construction of a 3 project in a manner that avoids causing erosion and/or the transport of pollutants off-site. 4
Appropriate Applications 5
Water conservation practices are implemented on all construction sites and 6 wherever water is used. 7
Water conservation practices apply to all construction projects. 8
Limitations 9
None identified. 10
Standards and Specifications 11
Keep water equipment in good working condition. 12
Stabilize the water truck filling area. 13
Repair water leaks promptly. 14
The washing of vehicles and equipment on the construction site is discouraged. 15
Avoid using water to clean construction areas. Do not use water to clean 16 pavement. Paved areas should be swept and vacuumed. 17
Direct construction water runoff to areas where it can infiltrate into the ground. 18
Apply water for dust control in accordance with the contract requirements. 19
Report discharges immediately. 20
Maintenance and Inspection 21
Inspect water equipment at least weekly. 22
Repair water equipment as needed. 23
7.5.2 DE-WATERING OPERATIONS 24
Definition and Purpose 25
De-watering operations are practices that manage the discharge of pollutants when non-26 stormwater and accumulated precipitation (stormwater) must be removed from a work 27 location so that construction work can be accomplished. 28
Appropriate Applications 29
These practices are implemented for discharges of non-stormwater and stormwater 30 (accumulated rain water) from construction sites. Non-stormwater includes, but is not 31
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limited to, groundwater, the de-watering of piles, water from cofferdams, water 1 diversions, and water used during construction activities that must be removed from a 2 work area. 3
Practices identified in this section are also appropriate for implementation when 4 managing the removal of accumulated precipitation (stormwater) from depressed areas 5 at a construction site. Stormwater mixed with non-stormwater should be managed as 6 non-stormwater. 7
Limitations 8
De-watering operations for non-stormwater will require and must comply with 9 applicable local permits, project-specific permits, and regulations. 10
Site conditions will dictate the design and use of de-watering operations. 11
A de-watering plan should be submitted as part of the SWPPP, detailing the 12 location of de-watering activities, equipment, and the discharge point. 13
The controls discussed in this BMP address sediment only. If the presence of 14 polluted water with hazardous substances is identified in the contract, the 15 contractor should implement de-watering pollution controls as required by the 16 contract documents. If the quality of water to be removed by de-watering is not 17 identified as polluted in the contract documents but is later determined by 18 observation or testing to be polluted, the contractor should comply with the 19 contract requirements. 20
Where possible, avoid de-watering discharges by using the water for dust 21 control, by infiltration, etc. 22
Standards and Specifications 23
De-watering for accumulated precipitation (stormwater) should follow this BMP 24 and use the treatment measures specified herein. 25
A project may require a separate National Pollutant Discharge Elimination 26 System (NPDES) permit prior to the de-watering discharge of non-stormwater. 27 These permits will have specific testing, monitoring, and discharge requirements 28 and can take a significant amount of time to obtain. 29
Monitoring for permit compliance will be required. 30
Discharges must comply with regional and watershed-specific discharge 31 requirements. 32
Additional permits or permissions from other agencies may be required for de-33 watering cofferdams or diversions. 34
De-watering discharges must not cause erosion at the discharge point. 35
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De-watering records should be maintained for a period of three years. 1
Maintenance and Inspection 2
Inspect all BMPs implemented frequently to ensure they comply with permit 3 requirements, and repair or replace BMPs as necessary to ensure they function 4 as designed. 5
Accumulated sediment removed during the maintenance of a de-watering device 6 may be incorporated into the project or disposed of outside of the project limits in 7 conformance with the contract requirements. 8
Accumulated sediment that is commingled with other pollutants must be 9 disposed of in accordance with all applicable laws and regulations. 10
Sediment Treatment 11
A variety of methods can be used to treat water during de-watering operations from the 12 construction site. Several devices are presented in this section that provide options to 13 achieve sediment removal. The sizes of particles present in the sediment and permit or 14 receiving water limitations on sediment are key considerations for selecting sediment 15 treatment options; in some cases, the use of multiple devices may be appropriate. 16
Category 1: Constructed Sediment Trap 17
Description 18
A sediment trap is a temporary basin with a controlled release structure that is 19 formed by excavation and/or construction of an embankment to detain sediment-20 laden runoff and allow sediment to settle out before discharging. 21
Appropriate Applications 22
A sediment trap is effective for the removal of trash, gravel, sand, and silt 23 and some metals that settle out with the sediment. 24
Implementation 25
The excavation and construction of related facilities is required. 26
Temporary sediment traps should be fenced if safety is a concern. 27
Outlet protection is required to prevent erosion at the outfall location. 28
Maintenance 29
Maintenance is required for safety fencing, vegetation, embankment, inlet 30 and outfall structures, as well as other features. 31
The removal of sediment is required when the storage volume is reduced 32 by one-third. 33
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Category 2: Mobile Settling Technologies 1
The devices discussed in this category are typical of tanks that can be used for the 2 sediment treatment of de-watering operations. A variety of vendors are available who 3 supply these tanks. 4
Weir Tank 5
Description 6
A weir tank separates water and waste by using weirs. The configuration of 7 the weirs (over and under weirs) maximizes the residence time in the tank 8 and determines the waste to be removed from the water, such as oil, grease, 9 and sediments. 10
Appropriate Applications 11
The tank removes trash, some settleable solids (gravel, sand, and 12 silt), some visible oil and grease, and some metals (removed with 13 sediment). To achieve high levels of flow, multiple tanks can be used 14 in parallel. If additional treatment is desired, the tanks can be placed 15 in a series or as pre-treatment for other methods. 16
Implementation 17
Tanks are delivered to the site by the vendor, who can provide 18 assistance with set-up and operation. 19
Tank size will depend on flow volume, constituents of concern, and 20 residency period required. Vendors should be consulted to 21 appropriately size tanks. 22
Maintenance 23
Periodic cleaning is required based on visual inspection or reduced 24 flow. 25
Oil and grease disposal must be performed by a licensed waste 26 disposal company. 27
28
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1 Figure 7-1: Weir Tank 2
De-Watering Tank 3
Description 4
A de-watering tank removes debris and sediment. Flow enters the tank 5 through the top, passes through a fabric filter, and is discharged through the 6 bottom of the tank. The filter separates the solids from the liquids. 7
Appropriate Applications 8
The tank removes trash, gravel, sand, and silt, some visible oil and 9 grease, and some metals (removed with sediment). To achieve high 10 levels of flow, multiple tanks can be used in parallel. If additional 11 treatment is desired, the tanks can be placed in a series or as pre-12 treatment for other methods. 13
Implementation 14
Tanks are delivered to the site by the vendor, who can provide 15 assistance with set-up and operation. 16
Tank size will depend on flow volume, constituents of concern, and 17 residency period required. Vendors should be consulted to 18 appropriately size the tank. 19
Maintenance 20
Periodic cleaning is required based on visual inspection or reduced 21 flow. 22
Oil and grease disposal must be performed by a licensed waste 23 disposal company. 24
25
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1 Figure 7-2: De-Watering Tanks 2
Category 3: Basic Filtration Technologies 3
Gravity Bag Filter 4
Description 5
A gravity bag filter, also referred to as a de-watering bag, is a square or 6 rectangular bag made of non-woven geotextile fabric that collects sand, silt, 7 and fines. 8
Appropriate Applications 9
Gravity bag filters are effective for the removal of sediments (gravel, 10 sand, and silt). Some metals are removed with the sediment. 11
Implementation 12
Water is pumped into one side of the bag and seeps through the 13 bottom and sides of the bag. 14
A secondary barrier, such as a rock filter bed or gravel bag barrier, is 15 placed beneath and beyond the edges of the bag to capture 16 sediments that escape the bag. 17
Maintenance 18
An inspection of the flow conditions, bag condition, bag capacity, and 19 the secondary barrier is required. 20
Replace the bag when it no longer filters sediment or passes water at 21 a reasonable rate. 22
The bag is disposed of off- or on-site. 23
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Category 4: Advanced Filtration Technologies 1
Sand Media Particulate Filter 2
Description 3
Water is treated by passing it through canisters filled with sand media. 4 Generally, sand filters provide a final level of treatment. They are often used 5 as a secondary or higher level of treatment after a significant amount of 6 sediment and other pollutants have been removed. 7
Appropriate Applications 8
Sand filters are effective for the removal of trash, gravel, sand, silt, 9 and some metals, as well as the reduction of Biochemical Oxygen 10 Demand (BOD) and turbidity. 11
Sand filters can be used for stand-alone treatment or in conjunction 12 with bag and cartridge filtration if further treatment is required. 13
Sand filters can also be used to provide additional treatment to water 14 treated through settling or basic filtration. 15
Implementation 16
The filters require delivery to the site and initial set-up. The vendor can 17 provide assistance with installation and operation. 18
Maintenance 19
The filters require constant service to monitor and maintain the sand media. 20
Pressurized Bag Filter 21
Description 22
A pressurized bag filter is a unit composed of single filter bags made from 23 polyester felt material. The water filters through the unit and is discharged 24 through a header, allowing for the flow to discharge in a series to an 25 additional treatment unit. Vendors provide pressurized bag filters in a variety 26 of configurations. Some units include a combination of bag filters and 27 cartridge filters for enhanced contaminant removal. 28
Appropriate Applications 29
A pressurized bag filter is effective for the removal of sediment (sand 30 and silt) and some metals, as well as the reduction of BOD, turbidity, 31 and hydrocarbons. Oil-absorbent bags are available for hydrocarbon 32 removal. 33
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Filters can be used to provide secondary treatment to water treated 1 through settling or basic filtration. 2
Implementation 3
The filters require delivery to the site and initial set-up. The vendor 4 can provide assistance with installation and operation. 5
Maintenance 6
The filter bags require replacement when the pressure differential 7 exceeds the manufacturer’s recommendation. 8
Cartridge Filter 9
Description 10
Cartridge filters provide a high degree of pollutant removal by using a number 11 of individual cartridges as part of a larger filtering unit. They are often used as 12 a secondary or higher (polishing) level of treatment after a significant amount 13 of sediment and other pollutants are removed. Units come with various 14 cartridge configurations (for use in a series with pressurized bag filters) or 15 with a larger single cartridge filtration unit (with multiple filters within). 16
Appropriate Applications 17
Cartridge filters are effective for the removal of sediment (sand, silt, 18 and some clays) and metals, as well as the reduction of BOD, 19 turbidity, and hydrocarbons. Hydrocarbons can effectively be removed 20 with special resin cartridges. 21
Filters can be used to provide secondary treatment to water treated 22 through settling or basic filtration. 23
Implementation 24
The filters require delivery to the site and initial set-up. The vendor 25 can provide assistance. 26
Maintenance 27
The cartridges require replacement when the pressure differential 28 exceeds the manufacturer’s recommendation. 29
7.5.3 TEMPORARY STREAM CROSSING 30
Definition and Purpose 31
A temporary stream crossing is a structure placed across a waterway that allows 32 vehicles to cross the waterway during construction, minimizing, reducing, or managing 33 the erosion and downstream sedimentation caused by the vehicles. 34
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Appropriate Applications 1
Temporary stream crossings are installed at sites: 2
Where appropriate permits have been secured (404 permits and 401 3 certification). 4
Where construction equipment or vehicles need to frequently cross a waterway. 5
When alternate access routes impose significant constraints. 6
When crossing perennial streams or waterways causes significant erosion. 7
Where construction activities will not last longer than one year. 8
Limitations 9
Temporary stream crossings will usually disturb the waterway during installation 10 and removal. 11
Temporary stream crossings will usually require 401 certification and a COE 404 12 permit. 13
Installation may require de-watering or a temporary diversion of the stream. 14
The crossings may become a constriction in the waterway, which can obstruct 15 flood flow and cause flow back-ups or washouts. If improperly designed, flow 16 back-ups can increase the pollutant load through washouts and scouring. 17
The use of natural or other gravel in the stream for the construction of a ford 18 crossing will be contingent upon approval by the appropriate agencies. 19
Ford crossings may degrade water quality due to contact with vehicles and 20 equipment. 21
Ford crossings should not be used in excessively high or fast flows. 22
Upon completion of construction activities, the crossing materials must be 23 removed from the stream. 24
25
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1
Figure 7-3: Typical Ford Crossing 2
Standards and Specifications 3
General Considerations 4
The location of the temporary stream crossing should address: 5
Site selection where erosion potential is low. 6
Areas where the side slopes from highway runoff will not spill into the side 7 slopes of the crossing. 8
The following types of temporary stream crossings should be considered: 9
Culverts: Used on perennial and intermittent streams. 10
Fords: Appropriate during the dry season in arid areas. Used on dry washes 11 and seasonal streams and low-flow perennial streams. Cellular Confinement 12 System (CCS), a type of ford crossing using manufactured blocks, is also 13 appropriate for use in streams. 14
Bridges: Appropriate for streams with high flow velocities, steep gradients, 15 and/or where temporary restrictions in the channel are not allowed. 16
The design and installation of a temporary stream crossing requires a knowledge of 17 stream flows and soil strength. Designs should be prepared under the direction of, 18 and approved by, a Registered Civil and/or Structural Engineer. Both hydraulic and 19 construction loading requirements should be considered: 20
The temporary stream crossing should comply with the requirements for 21 culvert and bridge crossings, particularly if it will remain through the rainy 22 season. 23
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The temporary stream crossing should provide stability in the crossing and 1 adjacent areas to withstand the design flow. The design flow and safety factor 2 should be selected based on careful evaluation of the risks due to 3 overtopping, flow back-ups, or washout. 4
The temporary stream crossing should avoid oil or other potentially 5 hazardous waste materials for surface treatment. 6
Construction Considerations 7
Stabilize construction roadways, adjacent work areas, and the stream bottom 8 against erosion. 9
Construct crossings during dry periods to minimize stream disturbance and 10 reduce costs. 11
Construct crossings at or near the natural elevation of the stream bed to 12 prevent potential flooding upstream of the crossing. 13
Install temporary sediment control BMPs in accordance with sediment control 14 BMPs to minimize the erosion of embankment into flow lines. 15
Vehicles and equipment should not be driven, operated, fueled, cleaned, 16 maintained, or stored in the wet or dry portions of a water body where 17 wetland vegetation, riparian vegetation, or aquatic organisms may be 18 destroyed, except as authorized by the RE, as necessary to complete the 19 work. 20
Temporary water body crossings and encroachments should be constructed 21 to minimize scour. Cobbles used for temporary water body crossings or 22 encroachments should be clean, rounded river cobble. 23
The exterior of vehicles and equipment that will encroach on the water body 24 within the project should be maintained free of grease, oil, fuel, and residues. 25
The disturbance or removal of vegetation should not exceed the minimum 26 necessary to complete operations. Precautions should be taken to avoid 27 damage to vegetation by people or equipment. Disturbed vegetation should 28 be replaced with the appropriate soil stabilization measures. 29
Riparian vegetation, when removed pursuant to the provisions of the work, 30 should be cut off no lower than ground level to promote rapid re-growth. 31 Access roads and work areas built over riparian vegetation should be 32 covered by a sufficient layer of clean river run cobble to prevent damage to 33 the underlying soil and root structure. The cobble should be removed upon 34 completion of project activities. 35
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Any temporary artificial obstruction placed within flowing water should only be 1 built from material such as clean gravel that will cause little or no siltation. 2
Drip pans should be placed under all vehicles, and equipment should be 3 placed on docks, barges, or other structures over water bodies when the 4 vehicle or equipment will be idle for more than one hour. 5
Specific Considerations 6
Culverts are relatively easy to construct and able to support heavy equipment 7 loads. 8
Fords are the least expensive of the crossings, with maximum load limits. 9
Temporary fords are not appropriate if construction will continue through the 10 rainy season, if thunderstorms are likely, or if the stream is perennial. 11
CCS crossing structures consist of clean, washed gravel and CCS blocks. 12 CCSs are appropriate for streams that would benefit from an influx of gravel 13 such as fish streams; streams or rivers below reservoirs; and urban, 14 channelized streams. Many urban stream systems are gravel-deprived due to 15 human influences such as dams, gravel mines, and concrete channels. 16
CCSs allow designers to use either angular or naturally-occurring, rounded 17 gravel, because the cells provide the necessary structure and stability. In fact, 18 natural gravel is optimal for this technique because of the habitat 19 improvement it will provide after removal of the CCS. 20
A gravel depth of 6 in. to 12 in. for a CCS structure is sufficient to support 21 most construction equipment. 22
An advantage of a CCS crossing structure is that relatively little rock or gravel 23 is needed because the CCS provides the stability. 24
Bridges are generally more expensive to design and construct but provide the 25 least disturbance of the stream bed and constriction of the waterway flows. 26
Maintenance and Inspection 27
Maintenance provisions should include: 28
The periodic removal of debris behind fords, in culverts, and under bridges. 29
The replacement of lost protective aggregate from inlets and outlets of 30 culverts. 31
The removal of temporary crossing promptly once it is no longer needed. 32
Inspection should, at a minimum, occur weekly and after each significant rainfall and 33 include: 34
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Checking for blockage in the channel, debris build-up in culverts, or behind 1 fords and under bridges. 2
Checking for erosion of abutments, channel scour, riprap displacement, or 3 piping in the soil. 4
Checking for structural weakening of the temporary crossing, such as cracks, 5 and undermining of foundations and abutments. 6
7.5.4 CLEAR WATER DIVERSION 7
Definition and Purpose 8
Clear water diversion consists of a system of structures and measures that intercept 9 clear surface water runoff upstream of a project site, transport it around the work area, 10 and discharge it downstream, with minimal water quality degradation for either the 11 project construction operations or construction of the diversion. Clear water diversions 12 are used in a waterway to enclose a construction area and reduce sediment pollution 13 from construction work occurring in or adjacent to water. Isolation techniques are 14 methods that isolate near shore work from a water body. Structures commonly used as 15 part of this system include diversion ditches, berms, dikes, slope drains, rock, gravel 16 bags, wood, sheet piles, aqua barriers, cofferdams, filter fabric or turbidity curtains, 17 drainage and interceptor swales, pipes, or flumes. 18
Appropriate Applications 19
A clear water diversion is typically implemented where appropriate permits (404 20 permits and 401 water quality certifications) have been secured and work must 21 be performed in a live stream or water body. 22
Clear water diversions are appropriate for isolating construction activities 23 occurring within or near a water body such as streambank stabilization or culvert, 24 bridge, pier, or abutment installation. They may also be used in combination with 25 other methods, such as clear water bypasses and/or pumps. 26
Pumped diversions are suitable for intermittent and low-flow streams. 27
Excavation of a temporary bypass channel or passing the flow through a pipe 28 called a flume is appropriate for the diversion of streams less than 20 ft. wide, 29 with flow rates less than 99 cu. ft. per second. 30
Limitations 31
Diversion/encroachment activities will usually disturb the waterway during the 32 installation and removal of diversion structures. 33
Specific permit requirements or mitigation measures, such as the USACE, 34 Federal Emergency Management Agency (FEMA), etc., may be included in 35 contract documents because of clear water diversion/encroachment activities. 36
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Diversion/encroachment activities may constrict the waterway, which can 1 obstruct flood flows and cause flooding or washouts. Diversion structures should 2 not be installed without identifying potential impacts to the stream channel. 3
Diversion or isolation activities should not completely dam stream flow. 4
De-watering and removal may require additional sediment control or water 5 treatment. 6
Standards and Specifications 7
General 8
Implement the guidelines presented for the streambank protection BMP. 9
Where working areas encroach on live streams, barriers adequate to prevent 10 the flow of muddy water into streams should be constructed and maintained 11 between working areas and streams. During construction of the barriers, the 12 muddying of streams should be held to a minimum. 13
Diversion structures must be adequately designed to accommodate 14 fluctuations in water depth or flow volume due to tides, storms, flash floods, 15 etc. 16
Heavy equipment driven in wet portions of a water body to accomplish work 17 should be completely clean of petroleum residue and water levels should be 18 below the gearboxes of the equipment in use, or lubricants and fuels should 19 be sealed so that an inundation of water does not result in leaks. 20
Mechanical equipment operated in the water should not be submerged to a 21 point above any axle of the mechanical equipment. 22
The excavation equipment buckets may reach out into the water for the 23 purpose of removing or placing fill materials. Only the bucket of the 24 crane/excavator/backhoe may operate in a water body. The main body of the 25 crane/excavator/backhoe should not enter the water body, except as 26 necessary to cross the stream to access the work site. 27
Stationary equipment such as motors and pumps, located within or adjacent 28 to a water body, should be positioned over drip pans. 29
When any artificial obstruction is being constructed, maintained, or placed in 30 operation, sufficient water should, at all times, be allowed to pass 31 downstream to maintain aquatic life downstream. 32
The exterior of vehicles and equipment that will encroach on a water body 33 within the project should be maintained free of grease, oil, fuel, and residues. 34
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Equipment should not be parked below the high-water mark, unless allowed 1 by a permit. 2
The disturbance or removal of vegetation should not exceed the minimum 3 necessary to complete operations. Precautions should be taken to avoid 4 damage to vegetation by people or equipment. Disturbed vegetation should 5 be replaced with the appropriate soil stabilization measures. 6
Riparian vegetation, when removed pursuant to the provisions of the work, 7 should be cut off no lower than ground level to promote rapid re-growth. 8
Access roads and work areas built over riparian vegetation should be 9 covered by a sufficient layer of clean river-run rock to prevent damage to the 10 underlying soil and root structure. The rock should be removed upon 11 completion of project activities. 12
Drip pans should be placed under all vehicles, and equipment should be 13 placed on docks, barges, or other structures over water bodies when the 14 vehicle or equipment is planned to be idle for more than one hour. 15
Where possible, avoid or minimize diversion/encroachment impacts by 16 scheduling construction during periods of low flow or when the stream is dry. 17 Also, see the project permits and contract requirements for scheduling. 18 Scheduling should also consider seasonal flows, fish spawning seasons, and 19 water demands due to irrigation. 20
Construct diversion structures with materials free of potential pollutants, such 21 as soil, silt, sand, clay, grease, or oil. 22
Temporary Diversions/Encroachments 23
Construct diversion channels in accordance with the guidelines for the earth 24 dikes, drainage swales, and ditches BMPs. 25
In high-flow velocity areas, stabilize the slopes of embankments and diversion 26 ditches using an appropriate liner, in accordance with the geotextiles, plastic 27 covers and erosion control blankets/mats BMPs, or use rock slope protection, as 28 described in the Standard Specifications. 29
Where appropriate, use natural stream bed materials such as large cobbles and 30 boulders for temporary embankment/slope protection or other temporary soil 31 stabilization methods. 32
Provide for velocity dissipation at transitions in the diversion, such as the point 33 where the stream is diverted to the channel and the point where the diverted 34 stream is returned to its natural channel, in accordance with the guidelines noted 35 under the outlet protection/velocity dissipation devices. BMPs. 36
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Temporary Dry Construction Areas 1
When de-watering behind temporary structures to create a temporary dry 2 construction area such as cofferdams, pass pumped water through a sediment 3 settling device, such as a portable tank or settling basin, before returning the 4 water to the water body. 5
If the presence of polluted water or sediment is identified in the contract, the 6 contractor should implement de-watering pollution controls as required by the 7 contract documents. If the quality of water or sediment to be removed while de-8 watering is not identified as polluted in the contract documents, but is later 9 determined by observation or testing to be polluted, the contractor should comply 10 with the contract requirements. 11
Any substance used to assemble or maintain diversion structures, such as form 12 oil, should be non-toxic and non-hazardous. 13
Any material used to minimize seepage underneath diversion structures, such as 14 grout, should be non-toxic, non-hazardous, and as close to a neutral pH as 15 possible. 16
Isolation Techniques 17
Isolation techniques are methods that isolate near-shore work from a water body. 18 Techniques include sheet pile enclosures, water-filled geotextile (aqua dam), gravel 19 berm with impermeable membrane, gravel bags, cofferdams, and K-rail. 20
Filter Fabric Isolation Technique 21
Definition and Purpose 22
A filter fabric isolation structure is a temporary structure built into a waterway to 23 enclose a construction area and reduce sediment pollution from construction 24 work in or adjacent to water. This structure is composed of filter fabric, gravel 25 bags, and steel T-posts. 26
Appropriate Applications 27
Geotextile filter fabric may be used for construction activities such as 28 streambank stabilization or culvert, bridge, pier, or abutment installation. It 29 may also be used in combination with other methods, such as clean water 30 bypasses and/or pumps. 31
This method involves the placement of gravel bags or continuous berms 32 to key in the fabric, subsequently staking the fabric in place. 33
If stream bed fish spawning gravel (gravel between 1 in. and 4 in.) is 34 used, all other components of the isolation can be removed from the 35 stream, and the gravel can be spread out and left as for the stream bed 36
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habitat. Whether spawning gravel or other types of gravel are used, only 1 clean, washed gravel should be used as infill for the gravel bags or 2 continuous berm. 3
This is a method that should be used in relatively calm water in smaller 4 streams. 5
Limitations 6
Do not use if the installation, maintenance, and removal of the structures 7 will disturb sensitive aquatic species of concern. 8
Filter fabric is not appropriate for projects where de-watering is 9 necessary. 10
Filter fabric is not appropriate to completely dam stream flow. 11
Standards and Specifications 12
For the filter fabric isolation method, a non-woven or heavy-duty fabric is 13 recommended over standard silt fence. Using rolled geotextile allows 14 non-standard widths to be used. 15
Anchor filter fabric with gravel bags filled with clean, washed gravel. Do 16 not use sand. If a bag should split open, the gravel can be left in the 17 stream, where it can provide aquatic habitat benefits. 18
Another anchor alternative is a continuous berm, made with the 19 continuous berm machine. This is a gravel-filled bag that can be made in 20 very long segments. The length of the berms is usually limited to 20 ft. for 21 ease of handling. 22
Installation 23
Place the fabric on the bottom of the stream, and place either a bag of 24 clean, washed gravel or a continuous berm over the bottom of the fabric, 25 so that a bag-width of fabric lies on the stream bottom. The bag should be 26 placed on what will be the outside of the isolation area. 27
Pull the fabric up and place a metal T-post immediately behind the fabric, 28 on the inside of the isolation area; and attach the fabric to the post with 29 three diagonal nylon ties. 30
Continue placing fabric as described above until the entire work area has 31 been isolated, staking the fabric at least every 6 ft. 32
Maintenance and Inspection 33
During construction, inspect daily during the work week. 34
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Schedule additional inspections during storm events. 1
Immediately repair any gaps, holes, or scour. 2
Remove sediment build-up. 3
Remove BMPs upon completion of all construction activity. Recycle or 4 reuse if applicable. 5
Re-vegetate areas disturbed by BMP removal if needed. 6
Turbidity Curtain Isolation Technique 7
Definition and Purpose 8
A turbidity curtain is a fabric barrier used to isolate the near-shore work area. The 9 barriers are intended to confine the suspended sediment. The curtain is a floating 10 barrier, and thus does not prevent water from entering the isolated area; rather, it 11 prevents suspended sediment from getting out. 12
Appropriate Applications 13
Turbidity curtains should be used where sediment discharge to a stream is 14 unavoidable. They are used when construction activities adjoin quiescent waters 15 such as lakes, ponds, lagoons, bays, and slow-flowing rivers. The curtains are 16 designed to deflect and contain sediment within a limited area and provide 17 sufficient retention time so that the soil particles will fall out of suspension. 18
Limitations 19
Turbidity curtains should not be used in flowing water; they are best 20 suited for use in ponds, lakes, lagoons, bays, and very slow-moving 21 rivers. 22
Turbidity curtains should not be placed across the width of a channel. 23
Removing sediment that has been deflected and settled out by the curtain 24 may create a discharge problem through the re-suspension of particles 25 and by accidental dumping by the removal equipment. 26
Standards and Specifications 27
Turbidity curtains should be oriented parallel to the direction of flow. 28
The curtain should extend the entire depth of the watercourse in calm-29 water situations. 30
In wave conditions, the curtain should extend to within 1 ft. of the bottom 31 of the watercourse, so that the curtain does not stir up sediment by hitting 32 the bottom repeatedly. If it is desirable for the curtain to reach the bottom 33
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in an active-water situation, a pervious filter fabric may be used for the 1 bottom 1 ft. 2
The top of the curtain should consist of flexible flotation buoys, and the 3 bottom should be held down by a load line incorporated into the curtain 4 fabric. The fabric should be a brightly-colored impervious mesh. 5
The curtain should be held in place by anchors placed at least every 6 100 ft. 7
First, place the anchors. Then tow the fabric out in a furled condition and 8 connect to the anchors. The anchors should be connected to the flotation 9 devices and not to the bottom of the curtain. Once in place, cut the furling 10 lines and allow the bottom of the curtain to sink. 11
Sediment that has been deflected and settled out by the curtain may be 12 removed. Consideration must be given to the probable outcome of the 13 removal procedure. Consider whether more of a sediment problem will be 14 created through re-suspension of the particles or by accidental dumping 15 of material during removal. It is recommended that the soil particles 16 trapped by the turbidity curtain only be removed if there has been a 17 significant change in the original contours of the affected area in the 18 watercourse. 19
Particles should always be allowed to settle for a minimum of 6 hours to 20 12 hours prior to their removal or prior to removal of the turbidity curtain. 21
Maintenance and Inspection 22
The curtain should be inspected daily for holes or other problems, and 23 any repairs needed should be made promptly. 24
Allow sediment to settle for 6 hours to 12 hours prior to the removal of 25 sediment or curtain. This means that after removing sediment, wait an 26 additional 6 hours to 12 hours before removing the curtain. 27
To remove, install furling lines along the curtain, detach from anchors, 28 and tow out of the water. 29
K-Rail (Pre-cast, Concrete Traffic Barriers) River Isolation 30
Definition and Purpose 31
This is a temporary sediment control, or stream isolation method, that uses K-32 rails to form the sediment deposition area, or to isolate the in-stream or near-33 bank construction area. Barriers are placed end-to-end in a pre-designed 34 configuration, and gravel-filled bags are used at the toe of the barrier and also at 35
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their abutting ends to seal and prevent the movement of sediment beneath or 1 through the barrier walls. 2
Appropriate Applications 3
K-rail isolation can be used in streams with higher water velocities than 4 many other isolation techniques. 5
Limitations 6
The K-rail method does not allow for full de-watering. 7
Standards and Specifications 8
To create a floor for the K-rail, move large rocks and obstructions. Place 9 washed gravel and gravel-filled bags to create a level surface for K-rail to 10 sit. 11
Place the bottom two K-rails adjacent to each other and parallel to the 12 direction of flow; fill the center portion with gravel bags. Then place the 13 third K-rail on top of the bottom two; there should be sufficient gravel bags 14 between the bottom K-rails, so that the top one is supported by the 15 gravel. Place plastic sheeting around the K-rails and secure at the bottom 16 with gravel bags. 17
Further support can be added by pinning and cabling the K-rails together. 18 Also, large riprap and boulders can be used to support either side of the 19 K-rail, especially where there is a strong current. 20
Maintenance and Inspection 21
The barrier should be inspected at least once daily, and any damage, 22 movement, or other problems should be addressed immediately. 23
Sediment should be allowed to settle for at least 6 hours to 12 hours prior 24 to removal of sediment and for 6 hours to 12 hours prior to removal of the 25 barrier. 26
Stream Diversions 27
Definition and Purpose 28
Stream diversions consist of a system of structures and measures that intercept 29 an existing stream upstream of the project, transports it around the work area, 30 and discharges it downstream. The selection of which stream diversion technique 31 to use depends upon the type of work involved, the physical characteristics of the 32 site, and the volume of water flowing through the project. 33
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Appropriate Applications 1
Pumped diversions are appropriate in areas where de-watering is 2 necessary. 3
Dam-type diversions may serve as temporary access to the site. 4
Diversions are appropriate in work areas that require isolation from flows. 5
Limitations 6
Pumped diversions have a limited flow capacity. 7
Pumped diversions require the frequent monitoring of pumps. 8
Large flows during storm events can overtop dams. 9
Flow diversion and re-direction with small dams involves in-stream 10 disturbance and the mobilization of sediment. 11
Standards and Specifications 12
Diversions should be sized to convey design flood flows. 13
Pump capacity must be sufficient for design flow; the upper limit is 14 approximately 10 cu. ft. per second (the capacity of two 8-in. pumps). 15
Adequate energy dissipation must be provided at the outlet to minimize 16 erosion. 17
Dam materials used to create dams upstream and downstream of the 18 diversion should be erosion-resistant; materials such as steel plate, sheet 19 pile, sandbags, continuous berms, inflatable water bladders, etc., would 20 be acceptable. 21
When constructing a diversion channel, begin excavation of the channel 22 at the proposed downstream end, and work upstream. Once the 23 watercourse to be diverted is reached and the excavated channel is 24 stable, breach the upstream end and allow water to flow down the new 25 channel. Once flow has been established in the diversion channel, install 26 the diversion weir in the main channel; this will force all water to be 27 diverted from the main channel. 28
Maintenance and Inspection 29
Inspect diversion/encroachment structures before and after significant 30 storms and at least once per week while in service. Inspect daily during 31 construction. 32
Pumped diversions require the frequent monitoring of pumps. 33
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Inspect embankments and diversion channels before and after significant 1 storms and at least once per week while in service for damage to the 2 linings, accumulating debris, sediment build-up, and the adequacy of 3 slope protection. Remove debris and repair linings and slope protection 4 as required. Repair holes, gaps, or scour. 5
In-Stream Construction Sediment Control 6
There are three different options currently available for reducing turbidity while 7 working in a stream or river. The stream can be isolated from the area in which work 8 is occurring with a water barrier; the stream can be diverted around the work site 9 through a pipe or temporary channel; or construction practices that minimize 10 sediment suspension can be used. 11
Whichever technique is implemented, an important thing to remember is that the 12 worst time to release high Total Suspended Solids (TSS) into a stream system might 13 be when the stream is very low. During these times, the flow may be low while the 14 biological activity in the stream is very high. Conversely, the addition of high TSS or 15 sediment during a big storm discharge might have a relatively low impact because 16 the stream is already turbid and the stream energy is capable of transporting both 17 suspended solids and large quantities of bedload through the system. The optimum 18 time to remove in-stream structures may be during the rising limb of a storm 19 hydrograph. 20
Techniques to Minimize Total Suspended Solids (TSS) 21
Padding: Padding laid in the stream below the work site may trap some 22 solids that are deposited in the stream during construction. After work is 23 done, the padding is removed from the stream and placed on the bank to 24 assist in re-vegetation. 25
Clean, washed gravel: Using clean, washed gravel decreases solid 26 suspension, as there are fewer small particles deposited in the stream. 27
Excavation using a large bucket: Each time a bucket of soil is placed in the 28 stream, a portion is suspended. Approximately the same amount is 29 suspended whether there is a small or large amount of soil placed in the 30 stream. Therefore, using a large excavator bucket will reduce the total 31 amount of soil that washes downstream. 32
Use of a dozer for backfilling: Using a dozer for backfilling instead of a 33 backhoe follows the same principles -- the fewer times soil is deposited in the 34 stream, the less soil will be suspended. 35
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Partial de-watering with a pump: Partially de-watering a stream with a 1 pump reduces the amount of water, and thus the amount of water that can 2 suspend sediment. 3
Washing Fines 4
Definition and Purpose 5
Washing fines is an in-channel sediment control method that uses water, either 6 from a water truck or hydrant, to wash any stream fines that were brought to the 7 surface of the channel bed during restoration back into the interstitial spaces of 8 the gravel and cobbles. 9
The purpose of this technique is to reduce or eliminate the discharge of sediment 10 from the channel bottom during the first seasonal flows, or first flush. Sediment 11 should not be allowed into stream channels; however, occasionally, in-channel 12 restoration work will involve moving or otherwise disturbing fines (sand and silt-13 sized particles) that are already in the stream, usually below bank-full discharge 14 elevation. Subsequent re-watering of the channel can result in a plume of 15 turbidity and sedimentation. 16
This technique washes the fines back into the channel bed. Bedload materials, 17 including gravel cobbles, boulders, and those fines, are naturally mobilized 18 during higher storm flows. This technique is intended to delay the discharge until 19 the fines would naturally be mobilized. 20
Appropriate Applications 21
This technique should be used when construction work is required in 22 channels. It is especially useful in intermittent or ephemeral streams in 23 which work is performed “in the dry” and is subsequently re-watered. 24
Limitations 25
The stream must have sufficient gravel and cobble substrate composition. 26
The use of this technique requires consideration of the time of year and 27 timing of expected stream flows. 28
The optimum time for the use of this technique is in the dry season. 29
Standards and Specifications 30
Apply water slowly and evenly to prevent runoff and erosion. 31
Consult with the Guam Environmental Protection Agency (GEPA) for 32 specific water quality requirements of applied water. 33
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1 Figure 7-4: In-Stream Erosion and Sediment Control Isolation Techniques 2
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1 Figure 7-5: Gravel Berm with Impermeable Membrane 2
3 Figure 7-6: Gravel Bag Technique 4
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1 Figure 7-7: In-Stream Erosion and Sediment Control Isolation 2
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1 Figure 7-8: K-Rail 2
7.5.5 POTABLE WATER/IRRIGATION 3
Definition and Purpose 4
Potable water/irrigation management consists of practices and procedures to manage 5 the discharge of potential pollutants generated during discharges from irrigation water 6 lines, landscape irrigation, lawn or garden watering, planned and unplanned discharges 7 from potable water sources, water line flushing, and hydrant flushing. 8
Appropriate Applications 9
Implement this BMP whenever the above activities or discharges occur at or enter a 10 construction site. 11
Limitations 12
None identified. 13
Standards and Specifications 14
Inspect irrigated areas within the construction limits for excess watering. Adjust 15 watering times and schedules to ensure that the appropriate amount of water is 16 being used and to minimize runoff. Consider factors such as soil structure, grade, 17 time of year, and type of plant material in determining the proper amounts of 18 water for a specific area. 19
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Equipment or other project washing activities runoff should be quality treated 1 prior to discharge to the storm drain or receiving water body. 2
Where possible, direct water from off-site sources around or through a 3 construction site in a way that minimizes contact with the construction site. 4
When possible, discharges from water line flushing should be reused for 5 landscaping purposes. 6
Shut off the water source to broken lines, sprinklers, or valves as soon as 7 possible to prevent excess water flow. 8
Protect downstream stormwater drainage systems and watercourses from water 9 pumped or bailed from trenches excavated to repair water lines. 10
Maintenance and Inspection 11
Repair broken water lines as soon as possible or as directed by the RE. 12
Inspect irrigated areas regularly for signs of erosion and/or discharge. 13
7.5.6 VEHICLE AND EQUIPMENT CLEANING 14
Definition and Purpose 15
Vehicle and equipment cleaning procedures and practices are used to minimize or 16 eliminate the discharge of pollutants from vehicle and equipment cleaning operations to 17 a storm drain system or to watercourses. 18
Appropriate Applications 19
These procedures are applied on all construction sites where vehicle and equipment 20 cleaning is performed. 21
Limitations 22
None. 23
Standards and Specifications 24
On-site vehicle and equipment washing is discouraged. 25
Cleaning vehicles and equipment with soap, solvents, or steam should not occur 26 on the project site unless the resulting wastes are fully-contained and disposed of 27 outside of the project limits in conformance with the contract requirements. 28 Resulting wastes and byproducts should not be discharged or buried within the 29 project limits and must be captured and recycled or disposed of. 30
Minimize the use of solvents. The use of diesel for vehicle and equipment 31 cleaning is prohibited. 32
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Vehicle and equipment washwater should be contained for percolation or 1 evaporative drying away from storm drain inlets or watercourses, and should not 2 be discharged within the project limits. Apply sediment control BMPs if 3 applicable. 4
All vehicles/equipment that regularly enter and leave the construction site must 5 be cleaned off-site. 6
When vehicle/equipment washing/cleaning must occur on-site, and the operation 7 cannot be located within a structure or building equipped with appropriate 8 disposal facilities, the outside cleaning area should have the following 9 characteristics: 10
o Located away from storm drain inlets, drainage facilities, or watercourses 11
o Paved with concrete or asphalt and bermed to contain washwaters and to 12 prevent run-on and run-off 13
o Configured with a sump to allow for the collection and disposal of 14 washwater 15
o Washwaters should not be discharged to storm drains or watercourses 16
o Used only when necessary 17
When cleaning vehicles/equipment with water: 18
o Use as little water as possible. High pressure sprayers may use less 19 water than a hose and should be considered. 20
o Use a positive shutoff valve to minimize water usage 21
o Facility wash racks should discharge to a sanitary sewer, recycle system, 22 or other approved discharge system and should not discharge to the 23 storm drainage system or watercourses 24
Maintenance and Inspection 25
The control measure should be inspected at a minimum of once a week 26
Monitor employees and subcontractors throughout the duration of the 27 construction project to ensure appropriate practices are being implemented 28
Inspect sump regularly and remove liquids and sediment as needed 29
7.5.7 VEHICLE AND EQUIPMENT FUELING 30
Definition and Purpose 31
Vehicle and equipment fueling procedures and practices are designed to minimize or 32 eliminate the discharge of fuel spills and leaks into storm drain systems or to 33 watercourses. 34
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Appropriate Applications 1
These procedures are applied on all construction sites where vehicle and equipment 2 fueling occurs. 3
Limitations 4
On-site vehicle and equipment fueling should only be used where it is impractical 5 to send vehicles and equipment off-site for fueling. 6
Standards and Specifications 7
When fueling must occur on-site, the contractor should select and designate an 8 area to be used. 9
Absorbent spill cleanup materials and spill kits should be available in fueling 10 areas and on fueling trucks and disposed of properly after use. 11
Drip pans or absorbent pads should be used during vehicle and equipment 12 fueling, unless the fueling is performed over an impermeable surface in a 13 dedicated fueling area. 14
Dedicated fueling areas should be protected from stormwater run-on and run-off, 15 and located at least 50 ft. from downstream drainage facilities and watercourses. 16 Fueling must be performed on level grade areas. 17
Nozzles used in vehicle and equipment fueling should be equipped with an 18 automatic shutoff to control drips. Fueling operations should not be left 19 unattended. 20
Protect fueling areas with berms and/or dikes to prevent run-on, run-off, and to 21 contain spills. 22
Use vapor recovery nozzles to help control drips as well as air pollution where 23 required. Ensure the nozzle is secured upright when not in use. 24
Fuel tanks should not be "topped-off." 25
Vehicles and equipment should be inspected on each day of use for leaks. Leaks 26 should be repaired immediately, or problem vehicles or equipment should be 27 removed from the project site. 28
Absorbent spill cleanup materials should be available in fueling and maintenance 29 areas and used on small spills instead of hosing the area down or using burying 30 techniques. The spent absorbent material should be removed promptly and 31 disposed of properly. 32
Federal, state, and local requirements should be observed for any stationary 33 above-ground storage tanks. 34
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Mobile fueling of construction equipment throughout the site should be 1 minimized. Whenever practical, equipment should be transported to the 2 designated fueling area. 3
Maintenance and Inspection 4
Fueling areas and storage tanks should be inspected regularly. 5
Keep an ample supply of spill cleanup material on the site. 6
Immediately clean up spills and properly dispose of contaminated soil and 7 cleanup materials. 8
7.5.8 VEHICLE AND EQUIPMENT MAINTENANCE 9
Definition and Purpose 10
Vehicle and equipment maintenance involves procedures and practices to minimize or 11 eliminate the discharge of pollutants to the storm drain systems or to watercourses from 12 vehicle and equipment maintenance procedures. 13
Appropriate Applications 14
These procedures are applied on all construction projects where an on-site yard area is 15 necessary for the storage and maintenance of heavy equipment and vehicles. 16
Limitations 17
None identified. 18
Standards and Specifications 19
Drip pans or absorbent pads should be used during vehicle and equipment 20 maintenance work that involves fluids, unless the maintenance work is performed 21 over an impermeable surface in a dedicated maintenance area. 22
All maintenance areas are required to have spill kits and/or use other spill 23 protection devices. 24
Dedicated maintenance areas should be protected from stormwater run-on and 25 run-off and located at least 50 ft. from downstream drainage facilities and 26 watercourses. 27
Drip pans or plastic sheeting should be placed under all vehicles and equipment 28 placed on docks, barges, or other structures over water bodies when the vehicle 29 or equipment is planned to be idle for more than one hour. 30
Absorbent spill cleanup materials should be available in maintenance areas and 31 disposed of properly after use. Substances used to coat asphalt transport trucks 32 and asphalt-spreading equipment should be non-toxic. 33
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Use off-site maintenance facilities whenever practical. For long-term projects, 1 consider constructing roofs or using portable tents over maintenance areas. 2
Properly dispose of used oils, fluids, lubricants, and spill cleanup materials. 3
Do not dump fuels and lubricants onto the ground. 4
Do not place used oil in a dumpster or pour into a storm drain or watercourse. 5
Properly dispose or recycle used batteries. 6
Do not bury used tires. 7
Repair fluid and oil leaks immediately. 8
Provide spill containment dikes or secondary containment around stored oil and 9 chemical drums. 10
Maintenance and Inspection 11
Maintain waste fluid containers in leak-proof condition. 12
Vehicle and equipment maintenance areas should be inspected regularly. 13
Vehicles and equipment should be inspected on each day of use. Leaks should 14 be repaired immediately, or the problem vehicles or equipment should be 15 removed from the project site. 16
Inspect equipment for damaged hoses and leaky gaskets routinely. Repair or 17 replace as needed. 18
7.5.9 PILE-DRIVING OPERATIONS 19
Definition and Purpose 20
The construction and retrofit of bridges and retaining walls often includes driving piles for 21 foundation support and shoring operations. Driven piles are typically constructed of 22 concrete, steel, or timber. Driven sheet piles are used for shoring and cofferdam 23 construction. The proper control and use of equipment, materials, and waste products 24 from pile-driving operations will reduce the discharge of potential pollutants to the storm 25 drain system or watercourses. 26
Appropriate Applications 27
These procedures apply to construction sites near or adjacent to a watercourse or 28 groundwater where permanent and temporary pile-driving operations (impact and 29 vibratory) take place. 30
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Standards and Specifications 1
Use drip pans or absorbent pads during vehicle and equipment maintenance, 2 cleaning, fueling, and storage. Refer to the BMPs “Vehicle and Equipment 3 Fueling” and “Vehicle and Equipment Maintenance.” 4
Have spill kits and cleanup materials available at all pile-driving locations. 5
Keep equipment that is in use in streambeds or on docks, barges, or other 6 structures over water bodies, leak free. 7
Where possible, park equipment over plastic sheeting or equivalent. Plastic 8 sheeting is not a substitute for drip pans or absorbent pads. The storage or use 9 of equipment in streambeds or other bodies of water should comply with all 10 applicable permits. 11
Implement other BMPs as applicable. 12
When not in use, store pile-driving equipment away from concentrated flows of 13 stormwater, drainage courses, and inlets. Protect hammers and other hydraulic 14 attachments from run-on by placing them on plywood and covering them with 15 plastic or a comparable material prior to the onset of rain. 16
Use less hazardous products, such as vegetable oil, instead of hydraulic fluid, 17 when practicable. 18
Maintenance and Inspection 19
Inspect pile-driving areas and equipment for leaks and spills on a daily basis. 20
Inspect equipment routinely and repair equipment as needed (e.g., worn or 21 damaged hoses, fittings, gaskets, etc.). 22
7.5.10 CONCRETE CURING 23
Definition and Purpose 24
Concrete curing is used in the construction of structures such as bridges, retaining walls, 25 and pump houses. Concrete curing includes the use of both chemical and water 26 methods. Proper procedures minimize runoff pollution during concrete curing. 27
Appropriate Applications 28
All concrete elements of a structure (e.g., footings, columns, abutments, stems, soffit, 29 decks, etc.) are subject to curing requirements. 30
Standards and Specifications 31
Chemical Curing 32
Avoid the overspray of curing compounds. 33
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Minimize the drift of chemical cure as much as possible by applying the 1 curing compound close to the concrete surface. Apply an amount of 2 compound that covers the surface but does not allow any runoff of the 3 compound. 4
Use proper storage and handling techniques for concrete curing compounds. 5
Protect drain inlets prior to the application of curing compounds. 6
Water Curing for Bridge Decks, Retaining Walls, and Other Structures 7
Direct curing water away from inlets and watercourses to collection areas for 8 removal in accordance with all applicable permits. 9
Collect cured water and transport or dispose of water in a non-erodible 10 manner. 11
Use wet blankets or a similar method that maintains moisture while 12 minimizing the use and possible discharge of water. 13
Maintenance and Inspection 14
Ensure that employees and subcontractors implement appropriate measures for 15 storage, handling, and the use of curing compounds. 16
Inspect any temporary diversion devices, lined channels, or swales for washouts, 17 erosion, or debris. Replace lining and remove debris as necessary. 18
Inspect cure containers and spraying equipment for leaks. 19
7.5.11 MATERIAL AND EQUIPMENT USE OVER WATER 20
Definition and Purpose 21
The following outlines procedures for the proper use, storage, and disposal of materials 22 and equipment on barges, boats, temporary construction pads, or similar locations that 23 minimize or eliminate the discharge of potential pollutants to a watercourse. 24
Appropriate Applications 25
These procedures should be implemented for construction materials and wastes (solid 26 and liquid) and any other materials that may be detrimental if released. These 27 procedures should be applied where materials and equipment are used on barges, 28 boats, docks, and other platforms over or adjacent to a watercourse. 29
Standards and Specifications 30
Use drip pans and absorbent materials for equipment and vehicles, and ensure 31 that an adequate supply of spill cleanup materials is available. Drip pans should 32 be placed under all vehicles and equipment placed on docks, barges, or other 33
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structures over water bodies when the vehicle or equipment is expected to be 1 idle for more than one hour. 2
Maintain equipment in accordance with the BMP, “Vehicle and Equipment 3 Maintenance.” 4
Provide watertight curbs or toe boards to contain spills and prevent materials, 5 tools, and debris from leaving the barge, platform, dock, etc. 6
Secure all materials to prevent discharges to receiving waters through wind. 7
Identify the types of spill control measures to be employed. Ensure that the staff 8 is trained to handle spills. 9
Ensure the timely and proper removal of accumulated wastes. 10
Comply with all necessary permits required for construction within or near the 11 watercourse. 12
Discharges to waterways should be reported immediately upon discovery. A 13 written discharge notification must follow within seven days. 14
Maintenance and Inspection 15
Inspect equipment for leaks and spills on a daily basis, and make necessary 16 repairs. 17
Ensure that employees and subcontractors implement appropriate measures for 18 the storage and use of materials and equipment. 19
7.5.12 CONCRETE FINISHING 20
Definition and Purpose 21
Concrete finishing methods are used for bridge deck rehabilitation, paint removal, curing 22 compound removal, and final surface finish appearances. Methods include sand 23 blasting, shot blasting, grinding, or high-pressure water blasting. Proper procedures 24 minimize the impact that concrete finishing methods may have on runoff. 25
Appropriate Applications 26
These procedures apply to all construction locations where concrete finishing operations 27 are performed. 28
Limitations 29
Specific permit requirements may be included in the contract documents for 30 certain concrete finishing operations. 31
Standards and Specifications 32
Follow the containment requirements stated in the contract documents, if any. 33
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Collect and properly dispose of water and solid waste from high-pressure water- 1 blasting operations. 2
Collect water from blasting operations, and transport or dispose of water in a 3 non-erodible manner. 4
Direct water from blasting operations away from inlets and watercourses to 5 collection areas for removal (e.g., de-watering), as approved in advance by the 6 RE and in accordance with applicable permits. 7
Protect inlets during sandblasting operations. 8
Minimize the drift of dust and blast material as much as possible by keeping the 9 blasting nozzle close to the surface. 10
Maintenance and Inspection 11
Follow the inspection procedures as required in the project’s contractual 12 specifications. 13
At a minimum, inspect containment structures, if any, for damage or voids prior to 14 their use each day and prior to the onset of rain. 15
At the end of each work shift, remove and contain the liquid and solid wastes 16 from containment structures, if any, and from the general work area. 17
7.5.13 STRUCTURE DEMOLITION/REMOVAL OVER OR ADJACENT TO 18 WATER 19
Definition and Purpose 20
This section outlines the procedures that should be followed to protect water bodies from 21 debris and wastes associated with structure demolition or removal over or adjacent to 22 watercourses. 23
Appropriate Applications 24
Structure demolition/removal over or adjacent to water should be applied in the cases of 25 full bridge demolition and removal, partial bridge removal (e.g., barrier rail, edge of deck) 26 associated with bridge-widening projects, concrete channel removal, or any other 27 structure removal that could potentially affect water quality. 28
Limitations 29
Specific permit requirements may be included in the contract documents. 30
Standards and Specifications 31
Do not allow demolished material to enter the waterway. 32
Refer to the BMP, “Clear Water Diversion”, to direct water away from work areas. 33
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Use attachments on construction equipment such as backhoes to catch debris 1 from small demolition operations. 2
Use covers or platforms to collect debris. 3
Platforms and covers should be approved by the RE. 4
Stockpile accumulated debris and waste generated during demolition away from 5 watercourses. 6
Ensure the safe passage of wildlife, as necessary. 7
Discharges to waterways should be reported to the RE immediately upon 8 discovery. A written discharge notification must follow within seven days. 9
Maintenance and Inspection 10
The contractor must inspect demolition areas over or near adjacent watercourses 11 on a daily basis. 12
Any debris-catching devices should be regularly emptied. Collected debris should 13 be removed and stored away from the watercourse and protected from run-on 14 and run-off. 15
7.6 POST-CONSTRUCTION STORMWATER MANAGEMENT BMPS 16
Maintenance BMPs are pollution prevention BMPs designed to reduce the discharge of 17 pollutants associated with maintenance activities. Maintenance BMPs apply to ongoing 18 maintenance of existing roadways, newly constructed BMPs and facilities, and other 19 facilities owned or operated by DPW. A list of BMPs is provided in Table 7-9. 20
21
Post-Construction Stormwater Management BMPs
Scheduling and Planning
Sediment Control
Silt Fence
Sandbag or Gravel Bag Barrier
Straw Bale Barrier
Fiber Rolls
Check Dam
Concentrated Flow Conveyance Controls
Over Side/Slope Drains
Ditches, Berms, Dikes, and Swales
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Post-Construction Stormwater Management BMPs
Temporary Diversion Ditches
Soil Stabilization
Compaction
Wood Mulch
Hydraulic Mulch
Hydroseeding/Handseeding
Straw Mulch
Clear-water Diversion
Work in a Water Body
Sediment Tracking Control
Tire Inspection and Sediment Removal
Waste Management
Spill Prevention and Control
Solid Waste Management
Hazardous Waste Management
Contaminated Soil Management
Sanitary/Septic Waste Management
Liquid Waste Management
Concrete Waste Management
Materials Handling
Material Delivery and Storage
Material Use
Vehicle and Equipment Operations
Vehicle and Equipment Fueling
Vehicle and Equipment Maintenance
Paving Operations Procedures
Water Conservation Practices
Potable Water/Irrigation
Safer Alternative Products
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Post-Construction Stormwater Management BMPs
Drainage Facilities
Baseline Stormwater Drainage Facilities Inspection and Cleaning
Enhanced Storm Drain Inlet Inspection and Cleaning Program
Illicit Connection Detection, Reporting, and Removal
Litter and Debris
Litter and Debris
Anti-Litter Signs
Chemical Vegetation Control
Vegetated Slope Inspection
Snow Removal and De-icing Agents
Dewatering Operations (Temporary Pumping Operations)
Sweeping and Vacuuming
Maintenance Facility Housekeeping Practices
Table 7-9: Maintenance BMPs 1 2
Potential pollutants of concern for DPW’s maintenance activities include petroleum products, 3 sediments, trash and debris, metals, acidic/basic materials, nutrients, solvents, waste paint, 4 herbicides, pesticides, and others. Many of these potential pollutants can be prevented from 5 being discharged through stormwater drainage systems by selecting and implementing 6 BMPs appropriate for the activity and subtask being conducted. 7
The majority of maintenance activities are performed in dry weather to minimize impacts to 8 water quality; however, conditions may exist which require some activities to be conducted 9 during wet weather. 10
11
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BIBLIOGRAPHY - 1 August 2010
BIBLIOGRAPHY 1
2
1. Arcement, G. J. and V. R. Schneider, 1984. Guide for Selecting Manning’s 3 Roughness Coefficients for Natural Channels and Flood Plains: Metric Version, 4 Water Supply Paper 2339, U.S. Geological Survey. Note: WSP2339 is the USGS 5 version of the U.S. Dept. of Transportation, Federal Highway Administration 6 Publication No. FHWA-TS-84-204, which has the same title. 7
2. Bradley, J.B., et al., October, 2005. Debris Control Structures – Evaluations and 8 Countermeasures, 3rd ed., Hydraulic Engineering Circular 9, Publication 9 No. FHWA-IF-04-016, U.S. Department of Transportation, Federal Highway 10 Administration, National Highway Institute, and Office of Bridge Technology. 11
3. Bradley, Joseph N., March, 1978. Hydraulics of Bridge Waterways, Hydraulic 12 Design Series 1, U.S. Department of Transportation, Federal Highway 13 Administration, Office of Engineering and Operations, Bridge Division. 14
4. Brown, S. A., et al., September, 2009. Urban Drainage Design Manual, 3rd ed., 15 Hydraulic Engineering Circular 22, Publication No. FHWA-NHI-10-009, U.S. 16 Department of Transportation, Federal Highway Administration, National Highway 17 Institute. 18
5. Brown, Scott. A. and Eric S. Clyde, March 1989. Design of Riprap Revetment, 19 Hydraulic Engineering Circular 11, Publication No. FHWA-IP-89-016, U.S. 20 Department of Transportation, Federal Highway Administration, Office of 21 Implementation: Research, Development, and Technology. 22
6. California Department of Transportation, May, 2007. Storm Water Quality 23 Handbooks-Project Planning and Design Guide. 24
7. California Department of Transportation, May, 2003. Statewide Stormwater 25 Management Plan, Publication No. CTSW-RT-02-008. 26
8. Commonwealth of Virginia, adopted April, 2002. Drainage Manual, Virginia 27 Department of Transportation, Location and Design Division, Hydraulics Section. 28
9. Cronshey, Roger, et al., revised June, 1986. Urban Hydrology for Small 29 Watersheds, Technical Release-55. Update of Appendix A, January, 1999; first 30 issued by the Soil Conservation Service, January, 1975. U.S. Department of 31 Agriculture, Natural Resources Conservation Service, Conservation Engineering 32 Division, Hydrology Unit. 33
10. Doubt, Paul D. and Gerald E. Oman, July, 1955. Routing through Tide Gates, 34 Technical Release No. 1. U.S. Department of Agriculture, Soil Conservation 35 Service, Engineering Division, Design Section. 36
BIBLIOGRAPHY - 2 August 2010
11. Douglass, Scott L. and Joe Krolak, June, 2008. Highways in the Coastal 1 Environment, 2nd ed., Hydraulic Engineering Circular 25, Publication No. FHWA-2 NHI-07-096, U.S. Department of Transportation, Federal Highway Administration, 3 National Highway Institute, and Office of Bridge Technology. 4
12. EarthTech, August, 1997. Flood Control Master Plan for Northern Guam: Phase 1, 5 prepared for Government of Guam Department of Public Works. 6
13. Engineering Publications, et al., June, 2008. Highway Runoff Manual, Publication 7 No. M 31-16.01, Washington Department of Transportation. 8
14. Engineering Publications, et al., March, 2007. Hydraulics Manual, 2nd ed., 9 Publication No. M 23-03.01, Washington Department of Transportation. 10
15. Guam Environmental Protection Agency, January, 2010. Guam Erosion Control and 11 Stormwater Management Draft Regulations. 12
16. Hawkins, Edward, et al., October, 1973. Hydrologic Engineering for Water 13 Resources Development, Vol. 4: Hydrograph Analysis, “Hydrologic Data 14 Management,” U.S. Army Corps of Engineers, Institute for Water Resources, 15 Hydrologic Engineering Center – International Hydration Decade. 16
17. Hjelmfelt, Allen T. Moody and the Natural Resources Conservation 17 Service/Agricultural Research Service, July, 2004. Hydrology National Engineering 18 Handbook, Part 630: Hydrology, Ch. 10, Estimation of Direct Runoff from Storm 19 Rainfall, Publication No. 210-VI-NEH. Originally prepared by Victor Mockus, 1964; 20 revised in 1998 by Helen Fox Moody. U.S. Department of Agriculture. 21
18. Holtz, PhD, Robert D., et al., revised April, 1998. Geosynthetic Design and 22 Construction Guidelines: Participant Notebook, Publication No. FHWA HI-95-038, 23 U.S. Department of Transportation, Federal Highway Administration, National 24 Highway Institute. 25
19. Horsley Witten Group, Inc., October, 2006. CNMI and Guam Stormwater 26 Management Manual, Vol. 1: Final – October 2006, prepared for Commonwealth of 27 the Northern Mariana Islands and the Territory of Guam. 28
20. Horsley Witten Group, Inc., October, 2006. CNMI and Guam Stormwater 29 Management Manual, Vol. 2: Final – October 2006, prepared for Commonwealth of 30 the Northern Mariana Islands and the Territory of Guam. 31
21. Johnson, Frank L. and Fred F.M. Chang, March, 1984. Drainage of Highway 32 Pavements, Hydraulic Engineering Circular 12, Publication No. FHWA-TS-84-202, 33 U.S. Department of Transportation, Federal Highway Administration, Office of 34 Implementation: Engineering & Highway Operations. 35
36
BIBLIOGRAPHY - 3 August 2010
22. Jones, J. Sterling, December, 2006. Effects of Inlet Geometry on Hydraulic 1 Performance of Box Culverts, Publication No. FHWA-HRT-06-138, U.S. Department 2 of Transportation, Federal Highway Administration, Office of Infrastructure Research 3 and Development, South Dakota Dept. of Transportation. 4
23. Kent, K. M., revised April, 1973. A Method for Estimating Volume and Rate of 5 Runoff in Small Watersheds, Publication No. SCS-TP-149, U.S. Department of 6 Agriculture, Soil Conservation Service, Hydrology Branch. 7
24. Kilgore, Roger T. and George K. Cotton, September, 2005. Design of Roadside 8 Channels with Flexible Linings, 3rd ed., Hydraulic Engineering Circular 15, 9 Publication No. FHWA-NHI-05-114, U.S. Department of Transportation, Federal 10 Highway Administration, National Highway Institute, and Office of Bridge 11 Technology. 12
25. Kornel Kerenyi, J., et al., March, 2007. Junction Loss Experiments: Laboratory 13 Report, Publication No. FHWA-HRT-07-036, U.S. Department of Transportation, 14 Federal Highway Administration, Office of Infrastructure Research and 15 Development. 16
26. Lagasse, P.F., et al., March, 2001. Stream Stability at Highway Structures, 3rd ed., 17 Hydraulic Engineering Circular 20, Publication No. FHWA-NHI-01-002, U.S. 18 Department of Transportation, Federal Highway Administration, National Highway 19 Institute, and Office of Bridge Technology. 20
27. Lagasse, P.F., et al., September, 2009. Bridge Scour and Stream Instability 21 Countermeasures Experience, Selection and Design Guidance, Vols. 1 and 2, 3rd 22 ed., Hydraulic Engineering Circular 23, Publication No. FHWA-NHI-09-11., U.S. 23 Department of Transportation, Federal Highway Administration, National Highway 24 Institute, and Office of Bridge Technology. 25
28. Masch, Frank D., October, 1984. Hydrology, Hydraulic Engineering Circular 19, 26 Publication No. FHWA-1P-84-15, U.S. Department of Transportation, Federal 27 Highway Administration, Office of Implementation. 28
29. McCuen, Richard, et al., October, 2002. Highway Hydrology: Hydraulic Design 29 Series 2, 2nd ed., Publication No. FHWA-NHI-02-001, U.S. Department of 30 Transportation, Federal Highway Administration, National Highway Institute. 31
30. Merkel, William H., September, 2008. Rainfall-Frequency and Design Rainfall 32 Distribution for Selected Pacific Islands, Technical Note No. 3, U.S. Department of 33 Agriculture, Natural Resources Conservation Services. 34
35
36
BIBLIOGRAPHY - 4 August 2010
31. Moody, Helen Fox and the Natural Resources Conservation Service/Agricultural 1 Research Service, July, 2004. Originally prepared by Victor Mockus, 1964. 2 Hydrology National Engineering Handbook, Part 630: Hydrology, Ch. 9, Hydrologic 3 Soil-Cover Complexes, Publication No. 210-VI-NEH, U.S. Department of 4 Agriculture. 5
32. Normann, Jerome M., et al., May, 2001 (revised May, 2005). Hydraulic Design of 6 Highway Culverts, 2nd ed., Hydraulic Design Series 5, Publication No. FHWA-NHI-7 01-020, U.S. Department of Transportation, Federal Highway Administration, 8 National Highway Institute, and Office of Bridge Technology. 9
33. Richardson, E.V. and S. R. Davis, May, 2001. Evaluating Scour at Bridges, 4th ed., 10 Hydraulic Engineering Circular 18, Publication No. FHWA NHI 01-001, U.S. 11 Department of Transportation, Federal Highway Administration, National Highway 12 Institute, and Office of Bridge Technology. 13
34. Richardson, E.V., et al., December, 2001. River Engineering for Highway 14 Encroachments, Highways in the River Environment, Hydraulic Design Series 6, 15 Publication No. FHWA-NHI-01-004, U.S. Department of Transportation, Federal 16 Highway Administration, National Highway Institute, and Office of Bridge 17 Technology. 18
35. Schall, James D., et al., June, 2008. Introduction to Highway Hydraulics, 4th ed., 19 Hydraulic Design Series 4, Publication No. WA-NHI-08-090, U.S. Department of 20 Transportation, Federal Highway Administration, National Highway Institute, and 21 Office of Bridge Technology. 22
36. Smith, Peter M., February, 2001. Highway Stormwater Pump Station Design, 23 Hydraulic Engineering Circular 24, Publication No. FHWA-NHI-01-007, U.S. 24 Department of Transportation, Federal Highway Administration, National Highway 25 Institute, and Office of Bridge Technology Applications. 26
37. Suphunvorranop, Thirasak, July, 1985. A Guide to SCS Runoff Procedures, 27 Technical Publication No. 85-5, Project No. 15/20-200-03, Department of Water 28 Resources, St. Johns River Water Management District. 29
38. Tenorio, Juan C. and Associates, September, 1980. Guam Storm Drainage Manual 30 – Guam Comprehensive Study: A Technical Report from the Comprehensive Study 31 of Guam’s Water and Related Land Resources, U.S. Army Corps of Engineers, 32 Honolulu District, Hawaii. 33
39. Thompson, Edward F. and Norman W. Scheffner, January, 2002. Typhoon-Induced 34 Stage-Frequency and Overtopping Relationships for the Commercial Port Road, 35 Territory of Guam: Final Report, Publication No. ERDC/CHL TR-02-1, U.S. Army 36 Corps of Engineers, Engineer Research and Development Center, Coastal and 37 Hydraulics Laboratory. 38
BIBLIOGRAPHY - 5 August 2010
40. Thompson, Philip L. and Roger T. Kilgore, July 2006. Hydraulic Design of Energy 1 Dissipators for Culverts and Channels, 3rd ed., Hydraulic Engineering Circular 14, 2 Publication No. FHWA-HNI-06-086, U.S. Department of Transportation, Federal 3 Highway Administration, National Highway Institute, and Office of Bridge 4 Technology. 5
41. U.S. Department of Transportation, Federal Highway Administration, Standard 6 Specifications for Construction of Roads and Bridges on Federal Highway Projects 7 (FP-03), U.S. Customary Units. 8
42. Woodward, Donald E., revised June, 1986. Time of Concentration, Technical Note 9 No. N4. Originally prepared by Paul I. Welle, February, 1983. U.S. Department of 10 Agriculture, Soil Conservation Service. 11
43. Woodward, Donald E. and the Natural Resources Conservation Service, March, 12 2007. Hydrology National Engineering Handbook, Part 630: Hydrology, Ch. 16, 13 Hydrographs, Publication No. 210-VI-NEH. Originally prepared by Dean Snider, 14 1971. U.S. Department of Agriculture. 15
44. Young, Fred Y. and Saku Nakamura, 1998. Soil Survey of Territory of Guam, U.S. 16 Department of Agriculture, Soil Conservation Service, in cooperation with the Guam 17 Department of Commerce and University of Guam. 18
45. Young, G. K., et al., May, 1993. Design of Bridge Deck Drainage, Hydraulic 19 Engineering Circular 21, Publication No. FHWA-SA-92-010, U.S. Department of 20 Transportation, Federal Highway Administration, Office of Technology Applications. 21
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APPENDICES
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APPENDIX I
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APPENDIX I
Temporary Erosion and Sediment Control Plans
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Appendix I TOC 1 August 2010
TABLE OF CONTENTS
TEMPORARY EROSION AND SEDIMENT CONTROL (TESC) PLAN........................................ 1
TESC REPORT OUTLINE ............................................................................................................ 3
CHAPTER 1. PROJECT OVERVIEW .......................................................................................... 3
1-1. Site Description .............................................................................................................. 3
1-1.1. Scope of Work ............................................................................................................ 3
1-1.2. Areas Impacted ........................................................................................................... 3
1-1.3. Existing Conditions ..................................................................................................... 3
1-1.4. Soils or Surface Conditions ......................................................................................... 4
1-1.5. Drainage ..................................................................................................................... 4
1-1.6. Off-Site Water ............................................................................................................. 4
1-1.7. Outfalls ........................................................................................................................ 4
1-1.8. Sensitive Areas ........................................................................................................... 4
1-1.9. Existing Water Quality ................................................................................................. 4
1-1.10. Affected Utilities .......................................................................................................... 4
1-1.11. Permits and associated reports .................................................................................. 4
1-1.12. Associated Reports ..................................................................................................... 5
1-2. TESC PLAN .................................................................................................................... 5
1-2.1. Construction Staging ................................................................................................... 5
1-2.2. Elements of TESC ...................................................................................................... 5
1-2.2.1. TESC Element 1: Mark Clearing Limits ................................................................... 5
1-2.2.2. TESC Element 2: Establish Construction Access ................................................... 5
1-2.2.3. TESC Element 3: Control Flow Rates ..................................................................... 6
1-2.2.4. TESC Element 4: Install Sediment Controls ............................................................ 7
1-2.2.5. TESC Element 5: Stabilize Soils ............................................................................. 7
1-2.2.6. TESC Element 6: Protect Slopes ............................................................................ 9
1-2.2.7. TESC Element 7: Protect Drain Inlets ................................................................... 10
1-2.2.8. TESC Element 8: Stabilize Channels and Outlets ................................................ 10
1-2.2.9. TESC Element 9: Control Pollutants ..................................................................... 10
1-2.2.10. TESC Element 10: Control De-Watering ............................................................... 11
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1-2.2.11. TESC Element 11: Maintain BMPs ....................................................................... 11
1-2.2.12. TESC Element 12: Manage the Project ................................................................ 11
1-3. Schedule ....................................................................................................................... 12
Appendix I 1 August 2010
TEMPORARY EROSION AND SEDIMENT CONTROL PLAN
The Temporary Erosion and Sediment Control (TESC) plan describes the temporary Best Management Practices (BMPs) selected for stormwater detention and water quality treatment during construction of the project. A BMP is a physical, structural, and/or managerial practice that prevents or reduces the pollution of water. The goal of the TESC plan is to prevent turbid discharges and sediments from leaving the site and to meet Guam’s Water Quality Standards. The TESC plan clearly establishes when and where specific BMPs will be implemented to prevent erosion and the transport of sediment from a site during construction. A TESC plan must address the 12 TESC elements, which are described later in this Appendix.
A TESC plan consists of a narrative section and plan sheets. The narrative section includes an analysis of pollution risk for each TESC element, as well as documentation to explain and justify the pollution prevention decisions made for the project. The TESC plan sheets show the BMP locations and other features, such as topography and sensitive area locations, for multiple project stages. It is recommended that large projects that will be under construction for multiple seasons create phased TESC plans, with one year of construction focusing on the wet seasons.
The following information must be included in the narrative:
Existing site conditions such as topography, drainage, soils, and vegetation
Potential erosion problem areas
BMPs chosen to address risks, as well as a list of applicable Federal Highway Administration (FHWA) Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects (FP-03) (Standard Specifications), the General Special Provisions (GSPs), and the Special Contract Requirements (SCRs).
Construction phasing/sequence and a general BMP implementation schedule (to be confirmed at a pre-construction meeting with the contractor)
The actions to be taken if BMP performance goals are not achieved
The contract plan sheets must include the following:
A north arrow
Right-of-way, project limits, and property boundaries
Existing topography including drainage, vegetation, structures, and roads
Top and bottom of slope catch lines for cut and fill slopes
Approximate slopes, contours, and the direction of stormwater flow before and after major grading activities
Appendix I 2 August 2010
Areas of soil disturbance and areas that will not be disturbed
Locations of structural and nonstructural pollution controls (temporary construction BMPs)
Locations of contractor staging including off-site material storage, stockpiles, waste storage, borrow areas, and vehicle/equipment storage/maintenance areas
Locations of all surface water bodies including wetlands, streams, and lakes, with the limits of their ordinary high water and buffer zones
Locations where stormwater or non-stormwater discharges off-site or to a surface water body
Locations of water quality sampling stations
Areas where final stabilization has been accomplished and no further construction-phase permit requirements apply
It is required that a TESC plan is prepared and signed by a Certified Erosion and Sediment Control Lead, and the Site Manager is trained and certified.
Appendix I 3 August 2010
TESC REPORT OUTLINE
Title Page: The following items should be included on the title page: the project number and name, associated route number and mileposts, name of report, date report was completed, designers’ names, and the project engineer’s professional civil engineers stamp and signature.
Table of Contents: Both the report and appendix contents should be listed in the Table of Contents.
CHAPTER 1. PROJECT OVERVIEW
The TESC plan describes the measures to be used during construction to protect the waters of Guam from degradation due to sediment transport or water pollution. These measures will require the use of temporary BMPs.
Field conditions during construction may require additional temporary BMPs or a change in placement of the temporary BMPs. The Contractor’s Erosion and Sediment Control Lead and the Department of Public Works’ (DPW’s) representatives should modify this plan if necessary to meet field conditions.
1-1. SITE DESCRIPTION
Besides the narrative, include a vicinity map that shows the project’s location on the island, as well as the right-of-way limits, project limits, existing roadways, proposed roadways, drainage basins, flow direction, location of nearby or adjacent construction activities, and sensitive areas (wetlands, streams, receiving water bodies, etc.).
1-1.1. SCOPE OF WORK
Describe the overall project activities and where they will occur (e.g., clearing and grubbing, grading, roadway excavation and embankment, constructing storm sewers, paving with asphalt concrete, constructing temporary BMPs, constructing permanent stormwater detention and infiltration ponds, bridge widening, adding turn lanes, constructing curb and gutter, landscape planters, sidewalks, construction of a bus pullout, fencing, pavement markings, illumination, signalization, traffic control, etc. Call out the construction plan sheets and details.
1-1.2. AREAS IMPACTED
Total area of new impervious surface added _____ sq. ft. or acres
Total area of disturbed soil _____ sq. ft. or acres
1-1.3. EXISTING CONDITIONS
This section presents the existing conditions on and surrounding the project site. Included are descriptions of soils, drainage, off-site water, outfalls, sensitive areas,
Appendix I 4 August 2010
existing water quality, Water Resource Inventory Areas (WRIAs), adjacent construction, and affected utilities.
1-1.4. SOILS OR SURFACE CONDITIONS
Identify the major soil type identified along the project. Identify other soil types along the project and rank by relative predominance.
1-1.5. DRAINAGE
Provide a list of stream names, locations, drainage pathways, and receiving water bodies.
1-1.6. OFF-SITE WATER
List the potential for off-site water to enter the project limits. State whether there is or is not the potential for off-site water. If applicable, include a discussion about the sources and waterways of off-site water that may enter the project site, and describe the measures to be taken, if any, to keep off-site water from mixing with on-site runoff. The site should be visited on a rainy day to verify conditions. State whether there is or is not an illicit connection that discharges within the project limits.
1-1.7. OUTFALLS
Include a table listing the milepost (and project stationing and control line offset as appropriate), condition, and location of each outfall within the project area.
1-1.8. SENSITIVE AREAS
Summarize the wetland report, areas of critical habitat, sensitive areas specified in basin plans, etc. If applicable, provide reference figures.
1-1.9. EXISTING WATER QUALITY
This section should include the Surface Water Classification for all nearby water bodies and information on impaired water bodies from the Guam Environmental Protection Agency (GEPA) assessment listing. Include the appropriate water quality standards.
1-1.10. AFFECTED UTILITIES
This section should include any utilities within the right-of-way of the proposed construction activities, which may be impacted by this construction project.
1-1.11. PERMITS AND ASSOCIATED REPORTS
Insert below the actual permits required for this project. If necessary, text describing why some permits may or may not be required should be added.
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1-1.12. ASSOCIATED REPORTS
Provide a list of reports. Revise to match actual reports and studies related to this project.
1-2. TESC PLAN
1-2.1. CONSTRUCTION STAGING
The narrative should include the description of construction staging and how the TESC will be provided for each stage. During the design phase, if it is impossible to know the timing and duration of a project, the TESC plans should be prepared assuming worst-case conditions for timing and duration.
1-2.2. ELEMENTS OF TESC
The 12 TESC elements are described below. All elements must be considered and included in the TESC plan, unless site conditions render an element unnecessary and the exemption is clearly justified in the narrative of the TESC plan. Common design and procedural BMPs are described for each element, followed by a list of physical BMPs, if applicable. Each element should be described using the following format:
Risk analysis
BMPs identified
GSPs
SCR
1-2.2.1. TESC Element 1: Mark Clearing Limits
Prior to land-clearing activities, mark all clearing limits on the plan and in the field with high-visibility fences to protect sensitive areas and their buffers (including vegetation to preserve), as well as adjacent properties. Retain duff layer, native topsoil, and existing vegetation in an undisturbed state to the maximum extent practicable.
Physical BMPs:
Preserving natural vegetation
Buffer zones
High-visibility fence
1-2.2.2. TESC Element 2: Establish Construction Access
Access points should be stabilized with a pad of quarry spalls, crushed rock, or equivalent BMP, in accordance with the Standard Specifications. Install stabilized construction access points prior to major grading operations. Limit
Appendix I 6 August 2010
access points to the fewest number possible, and only one, whenever feasible. Whenever practicable, slope entrances downward into the site to reduce track-out of sediments onto the roadway. If sediment is tracked off-site, roads should be cleaned thoroughly at the end of each day or more frequently, if necessary. Sediment should be removed from roads by shoveling or sweeping, and removed sediment should be transported to a controlled disposal area. If the stabilized construction entrance is not effective in preventing sediment from being tracked onto roads, a tire wash must be used and the wash water must be discharged to a separate on-site treatment system, such as closed-loop recirculation or land application, or discharged to a sanitary sewer (if allowed by individual permit).
Street sweeping is only allowed after sediment is removed from the street. If streets are washed with water, wash water must be treated prior to discharge.
Physical BMPs:
Stabilized construction entrance
Street sweeping
Construction road stabilization
Tire wash
1-2.2.3. TESC Element 3: Control Flow Rates
Protect downstream properties and waterways from erosion by preventing increases in the volume, velocity, and peak flow rate of stormwater runoff from the site during construction. Install the permanent sediment control facilities to provide flow control as early in the construction process as feasible. Protect infiltration facilities from siltation during the construction phase.
Install retention/detention facilities as one of the first steps in grading for use as infiltration or sedimentation facilities prior to mass grading and the construction of site improvements. Design drainages to account for both on- and off-site water sources. Use vegetated areas that are not identified as wetlands or other sensitive features to infiltrate and dispose of water whenever possible, and mark those areas on the TESC plan sheets.
Non-stormwater (de-watering, line flushing) discharges must also be controlled to protect downstream properties. When non-stormwater discharges are routed through separate storm sewer systems, the flow rate must be controlled to minimize scouring and the flushing of sediment trapped in the system.
Physical BMPs:
Temporary sediment pond
Appendix I 7 August 2010
Sediment trap
Stormwater infiltration
1-2.2.4. TESC Element 4: Install Sediment Controls
Install sediment control BMPs prior to soil-disturbing activities. Prior to leaving a construction site or discharging to an infiltration facility, concentrated stormwater runoff from disturbed areas must pass through sediment ponds or traps. Sheet flow runoff must pass through sediment control BMPs specifically designed to remove sediment from sheet flows, such as filter berms, vegetated filter strips, or silt fencing. Because maintaining sheet flows greatly reduces the potential for erosion, runoff should be maintained and treated as sheet flow whenever possible.
Physical BMPs:
Silt fence
Fiber Roll
Check dam
Temporary sediment pond or trap
Street sweeping
Surface roughening
Level spreader
Storm drain inlet protection
Outlet protection
Preserving natural vegetation
Portable storage water tanks
Vegetated filter strip
Stormwater chemical treatment
Filter berm (gravel, wood chip, or compost)
Construction stormwater filtration
1-2.2.5. TESC Element 5: Stabilize Soils
Stabilize all exposed and unworked soils by applying effective BMPs that protect the soil from wind, raindrops, and flowing water. Selected soil stabilization measures must be appropriate for the time of year, site conditions,
Appendix I 8 August 2010
estimated duration of use, and the water quality impacts that stabilization agents may have on downstream waters or groundwater.
Construction activity, including equipment staging areas, material storage areas, and borrow areas must be stabilized and addressed in the TESC plan as well.
Soil stockpiles are especially vulnerable to slumping when saturated and must be stabilized and protected with sediment-trapping measures. Plastic may be necessary on silty stockpiles, as it is the only BMP that can prevent soil saturation. Stockpiles should be located away from storm drain inlets, waterways, and drainage channels, where possible.
In Guam, cover exposed soil that is not being worked, whether at final grade or not, within the following time limits, using approved soil cover practices:
July 1st through November 30th – 2 days maximum
December 1st through June 30th – 7 days maximum
Immediately, in accordance with the rain forecast
Physical BMPs:
Preserving vegetation
Sodding
Temporary mulching
Check dam**
Soil binding using polyacrylamide*
Fiber Roll**
Placing erosion control blanket
Surface roughening***
Placing compost blanket
Stabilized construction entrance
Placing plastic covering
Construction road stabilization
Seeding and planting
Dust control BMPs
Gravel base
Bonded fiber matrix
Appendix I 9 August 2010
Mechanically-bonded fiber matrix
*While polyacrylamide alone does help stabilize soils, using it in conjunction with mulch provides more protection for disturbed soil.
**Check dams and fiber rolls alone do not stabilize soils. These BMPs should be used in conjunction with other soil stabilization BMPs.
***Surface roughening alone does not provide soil stabilization. Another BMP should be used in conjunction with surface roughening to protect the soil from raindrop impacts. It must be performed prior to seeding, in accordance with the Standard Specifications.
1-2.2.6. TESC Element 6: Protect Slopes
Design and construct cut-and-fill slopes in a manner that will minimize erosion by reducing the continuous length and steepness of slopes with terracing and diversions; reducing slope steepness; and roughening slope surfaces, considering soil type and its potential for erosion (such as track walking). In addition, all soil must be protected from concentrated flows through temporary conveyances such as diversions and pipe slope drains.
If flow is from seeps or groundwater, use best professional judgment when sizing slope drains. Conveyances exceeding a 10 percent slope should have a solid lining.
To capture sediment and runoff when cutting trenches, place excavated soil on the uphill side of the trench, when consistent with safety and space considerations.
Check dams should be placed at regular intervals within constructed channels that are cut down a slope.
Physical BMPs:
Channel lining (riprap, grass)
Subsurface drains
Temporary pipe slope drain
Level spreader
Temporary curb
Interceptor dike and swale
Gradient terraces
Physical BMPs listed under TESC Element 5, with the exception of stabilized entrance and road stabilization
Appendix I 10 August 2010
1-2.2.7. TESC Element 7: Protect Drain Inlets
Protect all operable storm drain inlets from sediment with approved inlet BMPs.
Physical BMPs:
Storm drain inlet protection (above/below grate and grate covers)
Check dam
1-2.2.8. TESC Element 8: Stabilize Channels and Outlets
All temporary conveyance channels should be designed, constructed, and stabilized to prevent erosion.
Stabilization, including armoring material, adequate to prevent erosion of outlets, adjacent stream banks, slopes, and downstream reaches should be provided at the outlets of all conveyance systems.
Physical BMPs:
Channel lining (riprap, grass)
Erosion control blanket
Level spreader
Sodding
Check dam
Outlet protection
Temporary seeding and planting
1-2.2.9. TESC Element 9: Control Pollutants
All pollutants, including construction materials, waste materials, and demolition debris, must be handled and disposed of in a manner that does not cause contamination of stormwater. Methods for controlling non-hazardous pollutants must be described in the TESC plan. Wood debris may be chopped and spread on-site. The application of fertilizers and other chemicals should be conducted in a manner and at application rates that will not result in loss of chemicals to stormwater runoff. Manufacturers’ label requirements for application rates and procedures must be followed.
Methods for controlling pollutants that can be considered hazardous materials, such as hydrocarbons and pH-modifying substances, must be described in the contractor’s Spill Prevention Control and Countermeasures (SPCC) plan.
Appendix I 11 August 2010
Stormwater or groundwater that has come into contact with curing concrete must be sampled to ensure Water Quality Standards are not violated. Process water (e.g., concrete washout, slurry water, and hydrodemolition) must be contained and cannot be discharged to the waters of Guam.
1-2.2.10. TESC Element 10: Control Dewatering
When groundwater is encountered in an excavation or other area, control, treat, and discharge it as described in the Standard Specifications.
1-2.2.11. TESC Element 11: Maintain BMPs
A Certified Erosion and Sediment Control Lead should inspect BMPs to ensure they perform their intended function properly until the Project Engineer determines that final stabilization is achieved. Final stabilization means the completion of all soil-disturbing activities and establishment of a permanent vegetative cover or permanent stabilization measures (such as riprap) to prevent erosion. Temporary BMPs should be removed within 30 days after final stabilization is achieved.
Maintain BMPs in accordance with the Standard Specifications. When the depth of accumulated sediment and debris reaches approximately one-third the height of the device, the contractor must remove the deposits. BMP implementation and maintenance should be documented in the Site Log Book. Clean sediments may be stabilized on-site.
1-2.2.12. TESC Element 12: Manage the Project
Apply the following actions on all projects:
1. Preserve vegetation and minimize disturbance and compaction of native soil, except as needed for building purposes.
2. Phase development projects to minimize the amount of soil exposed at any one time and prevent the transport of sediment from the site during construction.
3. Time sediment control BMP installation in accordance with TESC Element 4.
4. To minimize erosion, follow soil cover timing requirements and exposure limits in TESC Element 5 and the Standard Specifications. Projects that infiltrate all runoff are exempt from the above restrictions. Individual contract SCR and DPW directives may be more stringent based on specific location characteristics or changing site and weather conditions.
5. The work of utility contractors and subcontractors is coordinated to meet the requirements of both the TESC and SPCC plans.
Appendix I 12 August 2010
1-3. Schedule
A construction schedule must be provided by the contractor. The schedule should specify TESC plan implementation to effectively reduce erosion risks. Include the following in the schedule:
Installation of perimeter control and detention BMPs prior to soil-disturbing activities.
Phasing and timing of clearing, grubbing, and grading. Where feasible, work must be phased and timed to minimize the amount of exposed soil at any one time and prevent transport of sediment from the site during construction.
Application of interim BMP strategies when construction activities interfere with the placement of final-grade BMPs.
Discussion of how temporary BMPs are to be transitioned into permanent BMPs.
Implementation of an erosion control inspection and maintenance schedule.
APPENDIX IA
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APPENDIX IA
Correspondence with
Guam Environmental Protection Agency (GEPA)
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Appendix IA 1 August 2010
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APPENDIX II
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Appendix II
Hydraulic Report
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Appendix II TOC 1 August 2010
TABLE OF CONTENTS
HYDRAULIC REPORT .............................................................................................................................. 1
Procedure ................................................................................................................................................. 1
Review Copies ......................................................................................................................................... 1
Final Copies ............................................................................................................................................. 1
Hydraulic Report Revisions And Supplements ................................................................................... 1
Writing A Hydraulic Report .................................................................................................................... 1
HYDRAULIC REPORT OUTLINE ........................................................................................................... 1
CHAPTER 1. PROJECT OVERVIEW ................................................................................................... 1
1-1. Site Location ............................................................................................................................... 1
1-2. Vicinity Map ................................................................................................................................. 1
1-3. Scope Of Work ........................................................................................................................... 1
1-4. Areas Impacted .......................................................................................................................... 1
CHAPTER 2. EXISTING CONDITIONS ................................................................................................ 1
2-1. Existing Conditions .................................................................................................................... 1
2-2. Existing Hydraulic Features ...................................................................................................... 2
2-3. Existing Drainage Areas (DA) .................................................................................................. 2
2-4. Soils ............................................................................................................................................. 2
2-5. Outfalls ......................................................................................................................................... 2
2-6. Existing Utilities .......................................................................................................................... 2
CHAPTER 3. DEVELOPED CONDITIONS .......................................................................................... 2
3-1. Proposed Drainage Basins ....................................................................................................... 2
3-2. Design Standards ...................................................................................................................... 3
3-2.1. Design Frequency ...................................................................................................................... 3
3-2.2. Stormwater Management Guidelines ..................................................................................... 3
3-2.3. Other Requirements .................................................................................................................. 3
3-3. Pipe Alternatives ........................................................................................................................ 3
3-4. Downstream Analysis ................................................................................................................ 4
3-5. Hydrologic and Hydraulic Design ............................................................................................ 4
Appendix II TOC 2 August 2010
CHAPTER 4. PERMITS AND ASSOCIATED REPORTS .................................................................. 5
4-1. Environmental Issues, Fish, and Other Endangered Habitat .............................................. 5
4-2. Permits/Approvals ...................................................................................................................... 5
4-3. Right-of-Way and Easements .................................................................................................. 5
4-4. Additional Reports or Studies .................................................................................................. 6
CHAPTER 5. INSPECTION AND MAINTENANCE SUMMARY ....................................................... 6
CHAPTER 6. APPENDIX ........................................................................................................................ 6
6-1. Environmental Documentation ................................................................................................. 6
6-1.1. TESC Narrative and Plan Sheets ............................................................................................ 6
6-2. Basin Calculations and Exhibits............................................................................................... 6
6-3. Calculations and Program Output ........................................................................................... 6
6-4. Drainage Plan Sheets, Details, and Structure Notes ........................................................... 6
6-5. Drainage Profile Plan Sheets ................................................................................................... 6
6-6. Roadway Typical Cross Sections and Profiles ...................................................................... 7
6-7. Miscellaneous Contract Plan Sheets ...................................................................................... 7
Appendix II 1 August 2010
HYDRAULIC REPORT
PROCEDURE
Hydraulic reports should be submitted to the Department of Public Works (DPW) or their designated representative, as follows.
REVIEW COPIES
Designers should submit a complete hard copy of the Hydraulic Report to DPW or its designated representative for review.
FINAL COPIES
Upon approval, two paper copies of the Hydraulic Report, one electronic copy (CD) of the report, and the original approval letter should be sent to the following offices:
1. The Construction Office for reference during construction.
2. DPW’s Chief Engineer, who will keep it in a secure location as the record of the project stormwater management facilities.
HYDRAULIC REPORT REVISIONS AND SUPPLEMENTS
At times, a hydraulic report may need to be revised due to various elements within a proposed project. There are two ways to submit a change:
1. Revision: A revision is a correction of the existing report either due to an error or omitted design documentation. The designer should submit the revision along with a new title page, stamped and signed by the Project Engineer, showing both the original approval date and the revision date. The revised document should contain a statement immediately behind the cover page that provides a summary of the revisions.
2. Supplement: A supplement is a change that was not part of the original scope of work. The same approval process is required as with the original report; however, the supplement should be a stand-alone document that references the original report.
Either type of change should be prepared documenting the revisions made, with back-up documentation. Include revised plans, calculations, and other updates as warranted in a submittal package to DPW. An approval/concurrence letter will be issued for the supplement.
WRITING A HYDRAULIC REPORT
A Hydraulic Report Outline has been provided below as a starting point for designers. Organizing reports in the outline format will help to expedite the review process. It is recommended that designers read through the outline and determine which sections are applicable to their project. Any sections that are not applicable will include the heading, with the
Appendix II 2 August 2010
tern “N.A.” under the heading in the report. DPW or its designated representatives can be contacted for assistance in preparing a Hydraulic Report.
The Hydraulic Report should contain the elements listed in the outline that apply to the project. Designers should provide a well-organized report so that an engineer with no prior knowledge of the project could read and fully understand the hydraulic/hydrologic design of the project. The report should contain enough information to allow someone else to reproduce the design in its entirety, but at the same time, designers should be brief and concise, careful not to provide duplicate information that could create confusion.
Following and completing the outline does not mean that all of the minimum requirements in the Highway Runoff Manual have been addressed. The outline is only a tool to aid the designer in developing a Hydraulics Report. To determine the applicability of the minimum requirements and Best Management Practices (BMPs) for a project, designers should consult the other sections of this Guam Transportation Stormwater Drainage Manual (TSDM).
Appendix II 1 August 2010
HYDRAULIC REPORT OUTLINE
Title Page: The following items should be included on the title page: the project number and name, associated route number and mileposts, name of report, date report was completed, designers’ names, and both the project engineer’s professional civil engineers stamp and signature.
Table of Contents: Both the Hydraulic Report and appendix contents should be listed in the Table of Contents. Number all pages, including maps, figures, and tables both in the report and in the appendices.
CHAPTER 1. PROJECT OVERVIEW
1-1. SITE LOCATION
Note the following: milepost limits, section, township, and range and reference location from nearest village.
1-2. VICINITY MAP
Include a vicinity map with the project location on the island, clearly shown. Whenever possible, highlight major landmarks, delineate water bodies, and label cross streets. A vicinity map may require using two maps with different scales, one for the location on the island, and a second showing the main features of the project including the project limits, mileposts, and other main topographic features.
1-3. SCOPE OF WORK
Introduce the hydraulic features of the project and note why they are being installed. Describe project improvements and where they will occur, and reference attached plan sheets where applicable. It is not necessary to discuss the overall purpose of the project unless it is pertinent to some of the decisions made during the design of hydraulics features.
1-4. AREAS IMPACTED
List the following total areas (in acres): New impervious surfaces, replaced impervious surfaces, total existing impervious surfaces, and total areas being converted from native vegetation to lawn or landscaped (if applicable). Exhibits will need to be included in the report appendix to support the impervious/pervious area calculations.
CHAPTER 2. EXISTING CONDITIONS
2-1. EXISTING CONDITIONS
Include a discussion on the project site conditions and layout as observed during inspection of the site by the designer. The discussion should serve to confirm what is
Appendix II 2 August 2010
shown on the maps and site plans, as well as note any features that will influence the drainage design.
2-2. EXISTING HYDRAULIC FEATURES
Note any existing drainage features and describe how they operate prior to construction. Also note how project improvements could impact their operation and how they will function during post-construction activities. If needed, use photographs to describe the site. Identify any bridges within the project limits.
2-3. EXISTING DRAINAGE AREAS (DA)
For each DA within the project, provide a description of the general drainage systems and flow patterns. (See Chapter 4 for a detailed description of DAs). Unusual or unique drainage patterns should be discussed. These areas should also be part of the description. The designer should also list any off-site flows. Each DA description should list the eventual downstream water body. Drainage maps delineating entire DAs (i.e., beyond DPW’s right-of-way) should be included in the report appendices, as should individual basin area calculations. This section should refer to where the basin information is located in the appendices.
2-4. SOILS
Discuss the soil testing that has been performed at the site. This includes soil pH and resistivity to determine acceptable pipe alternatives, soil borings, soil type from National Resources Conservation Service maps, soil infiltration, groundwater level, etc., for stormwater BMP design.
2-5. OUTFALLS
An outfall can be any structure (man-made or natural) where stormwater from DPW highways is conveyed off the right-of-way. (If no stormwater outfalls leave the DPW right-of-way, the designer should note that instead).
2-6. EXISTING UTILITIES
Note utility conflicts that have been investigated and either are or are not an issue. If there is a conflict, please note the resolution. Utilities should be shown on the drainage plan and profile sheets.
CHAPTER 3. DEVELOPED CONDITIONS
3-1. PROPOSED DRAINAGE BASINS
This section should discuss and tabulate the proposed basins, including what has changed regarding pervious and impervious areas to the existing basins. Maps showing all DAs and drainage basin areas significant to the project (including portions that are off the DPW right-of-way) should be included in the report appendices along with all basin calculations. This section should serve to confirm what is shown on the final drainage
Appendix II 3 August 2010
plans, profiles, and details. Note that final level plans may not yet be completed but will be checked against the Hydraulic Report during review.
Drainage maps should show basin and sub-basin boundaries, areas, and flow direction arrows. Each drainage basin should be clearly labeled, with the same label referenced in the hydrologic and hydraulic calculations. When the change between existing and post-construction conditions is important to the calculations, the maps should show both conditions, on separate maps, if necessary, for clarity. Maps should always be of an adequate scale to allow reviewers to verify all basin calculations.
3-2. DESIGN STANDARDS
3-2.1. DESIGN FREQUENCY
Note the appropriate design frequencies used to size hydraulic features on the project, and where applicable, show calculations. Include a discussion of the climate and chosen precipitation values for the project.
3-2.2. STORMWATER MANAGEMENT GUIDELINES
State the stormwater management criteria and guidelines used for flow control, basic runoff treatment, and enhanced runoff treatment (if applicable). Include a summary table noting the BMP and the project location (including structure notes for each drainage feature) organized by alignment, drainage basin, station, and offset and/or milepost. The table should include DA-specific pre-project and post-project pervious and impervious surface areas. Also note what percentage of the new (and replaced, if applicable) impervious surface areas are addressed for flow control and runoff treatment according to the stormwater management criteria used for this report. Wherever possible, use the same structure note from the plans in the hydrologic and hydraulic calculations.
3-2.3. OTHER REQUIREMENTS
Note any additional requirements used in the hydraulic calculations that differ or are in addition to those found in the TSDM. Provide a list of references for the guidelines, manuals, basin plans, local agency codes, or technical documents used to develop the hydraulics report, and where possible, include a Web link to the reference.
3-3. PIPE ALTERNATIVES
Note all acceptable pipe alternatives for the project, and provide an engineering justification for any alternatives that are excluded. This should reference the pipe design life in relation to the various site conditions (soil type, pH, soil resisitivity, other corrosion potential and load requirements).
Appendix II 4 August 2010
3-4. DOWNSTREAM ANALYSIS
This section focuses on what impact a project will have on the hydraulic conveyance systems downstream and how the impact is mitigated. The analysis should be divided into three sections:
1. A review of the existing conditions
2. An analysis of the project impacts
3. Mitigation alternatives with detail on the selected alternative
3-5. HYDROLOGIC AND HYDRAULIC DESIGN
Hydrologic and hydraulic design calculations for all hydraulic features should be discussed and the results summarized in this section (e.g., culverts, storm drains, stormwater BMPs, inlets, gutters, ditches, stream bank stabilization). Where applicable, it is recommended that the design be divided into the sections described below. The locations of the items on the next page should also be noted, and where there are multiple locations, designers should consider using a table for clarity.
Calculations should include the following:
Drainage area quantities and assumptions with equivalent area trading
The equations used for the actual numerical calculations
A discussion of what assumptions were made to perform the calculations
How the input parameters were determined
The calculations should always include enough supporting information to allow reviewers to completely duplicate the process used throughout the original design; however, excessive data that duplicates information already provided can often make the calculation process less understandable. Whenever possible, the calculation methodologies described in the TSDM should be followed, including the following:
Figures from the TSDM
Standard design forms
Suggested software
If a different method or software is selected, the reason for not using the standard DPW method should be explained and approved prior to submitting the report. Actual calculations, design forms, exhibits, and output from software used in the project design should be included as part of the report appendices.
All calculations included in the report should be checked by an individual other than the person who prepared the report. The checker should be a person of equal or higher technical level as the designer. The checker should sign and date each sheet of the
Appendix II 5 August 2010
calculations. The calculation summaries should be included under the following outline sections:
3.5.1 Flow Control BMPs
3.5.2 Runoff Treatment BMPs
3.5.3 Gutter Design
3.5.4 Sag Design
3.5.5 Enclosed Drainage Design
3.5.6 Culvert Design
3.5.7 Ditch Design
3.5.8 Special Stream Design
3.5.9 FloodPlain Mitigation
3.5.10 Bridge Scour Evaluation
3.5.11 Channel Changes
3.5.12 Downstream Analysis
CHAPTER 4. PERMITS AND ASSOCIATED REPORTS
4-1. ENVIRONMENTAL ISSUES, FISH, AND OTHER ENDANGERED HABITAT
Describe any water quality receiving bodies, floodplains, stream crossings, wetlands, steep slopes, or other sensitive areas within the project limits, noting project impacts. Describe any fish passage design issues with culverts within the project limits. Note if fish surveys were conducted and what was determined. Also note if there are any threatened or endangered species within the project limits. This information is usually detailed in the project’s Environmental Assessment (EA) reports. Include only an overview summary of the various aspects in this section, and reference the EAs for the details.
4-2. PERMITS/APPROVALS
List all permits, variances, or approvals required by local jurisdictions and resource agencies that are necessary to complete the project. Categorize the permits that are obtained by DPW and those that must be obtained by the contractor for construction. Include a list of environmental commitments as they pertain to stormwater issues that are stated in the EAs, the project’s environmental impact statement, permits, and local regulations.
4-3. RIGHT-OF-WAY AND EASEMENTS
Note any additional easements that may be required for completion of the stormwater facilities on the project, noting whether the easement is for permanent facilities,
Appendix II 6 August 2010
installation and maintenance, or a temporary easement for contractor access or construction. Highlighting and referencing the areas on the attached plan sheets is helpful.
4-4. ADDITIONAL REPORTS OR STUDIES
Where applicable, note other reports and studies conducted and prepared for this project. Examples are the Geotechnical Report (used to note drainage-related special studies needed for the project, including, but not limited to, soil tests, infiltration rates, and well monitoring) and the environmental reports.
CHAPTER 5. INSPECTION AND MAINTENANCE SUMMARY
Maintenance should be consulted prior to starting a project concerning any existing drainage problems or requirements, including maintenance access.
CHAPTER 6. APPENDIX
6-1. ENVIRONMENTAL DOCUMENTATION
The purpose of this section is to document environmental decisions made during the design, including reasons why decisions were made, who made the decisions, and notes on any references used. If pollution-loading calculations are required by the regulatory agencies, these should be included here.
6-1.1. TESC NARRATIVE AND PLAN SHEETS
6-2. BASIN CALCULATIONS AND EXHIBITS
The drainage maps and any exhibits that show how the basins, sub-basins, and flows were calculated should be included along with the hydrology calculations, hydrographs, or other basin hydraulic modeling.
6-3. CALCULATIONS AND PROGRAM OUTPUT
Include all calculations used to create the hydraulics report, including software outputs, inlet calculation spreadsheets, sag analyses, etc. All calculations should be clearly labeled with a file name and initialed (and checked) by an individual other than the one who prepared the report.
6-4. DRAINAGE PLAN SHEETS, DETAILS, AND STRUCTURE NOTES
For culverts and bridge projects, include the Water Surface Elevation and design flow rates for the design frequency storms (usually ordinary high water, 25-year and/or 50-year, 100-year, 500-year and if tidal, mean low, mean high, and mean higher high-water elevations).
6-5. DRAINAGE PROFILE PLAN SHEETS
For culverts and bridge projects, include the Water Surface Elevation and design flow rates for the design frequency storms (usually ordinary high water, 25-year and/or 50-
Appendix II 7 August 2010
year, 100-year, 500-year, and if tidal, mean low, mean high, and mean higher high-water elevations). Plans and profiles should be prepared to 1 inch equals a 40-ft. horizontal scale, and 1 inch equals a 4-ft. vertical scale, unless specified otherwise.
6-6. ROADWAY TYPICAL CROSS SECTIONS AND PROFILES
The cross sections should be prepared to a scale of 1 inch equals a 10-ft. horizontal scale and 1 ft. equals a 5-ft. vertical scale, unless specified otherwise. Plans and profiles should be prepared to 1 inch equals a 40-ft. horizontal scale and 1 inch equals a 4-ft. vertical scale, unless specified otherwise.
6-7. MISCELLANEOUS CONTRACT PLAN SHEETS
Include any plan sheets that will aid the reviewer in completely understanding the project. This may include utility plan sheets.
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APPENDIX III
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Appendix III
Establishment of Intensity Duration Relationships
for Precipitation in Guam
for Design of Transportation-Related Facilities
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Appendix III 1 August 2010
INTRODUCTION
The objective of this report is to develop the rainfall Intensity Duration Frequency (IDF) relationship and design storm duration for design of roadway-related drainage facilities for the Island of Guam. The rainfall IDF relationship is one of the most commonly used tools in hydraulic engineering for planning, designing, and operations of various engineering projects. Engineers use precipitation-frequency information in making decisions concerning the size of stormwater management facilities, as stormwater management facilities must be sized to create a logical balance between the safety of the project and the economics of installation, maintenance, and replacement. Accordingly, different aspects of stormwater conveyance, quality treatment, and flow control are designed to different levels or frequencies of storm events. These design storm events are selected based on an updated statistical analysis of the Guam precipitation records.
A one-hour duration storm is recommended for the design of transportation stormwater facilities for Guam. The one-hour storm duration is selected from statistical analysis, which showed that most precipitation on Guam occurs during intense, short duration storms. The probability of getting storms longer than a one-hour duration, as well as having more than one one-hour storm in a day is statistically remote.
The one-hour storm distribution is a hybrid curve based on an analysis of the 15-minute rainfall from the partial duration statistical series, and the lesser 30- and 60-minute precipitation amounts from the one-hour storm series. The subsequent intensities for durations shorter than 15 minutes are somewhat conservative, while the longer duration intensities closely match the statistically-predicted rainfall depths. This report discusses in detail, the statistical analysis comparison of these statistical results to other statistical studies, and includes a discussion of the selection of design storms.
BACKGROUND
Despite its small size, Guam is characterized by one of the highest levels of rainfall variability in the world, with the highest annual rainfall nearly three times the amount of the lowest rainfall. There is a dry season from January to April and a rainy season from July to mid-November. The other months are transitional periods that may be either rainy or dry depending on the nature of the particular year. Exceptionally dry years recur about once every 4 years in correlation with episodes of El Nino Southern Oscillation (ENSO) in the Pacific. Mean annual rainfall ranges from 85 inches in western Guam to 115 inches on the upper slopes of the southern mountains. On average, approximately 55 percent of the rainfall occurs in the rainy season and 15 percent during the dry season, with the remainder falling during transitional months. The prevailing winds blow from the northeast or east for much of the year. The island is regularly affected by typhoons, which bring heavy winds and rains. Lander (2004) suggested that rainfall in Guam is not elevation dependent. Extreme rainfall events in Guam are almost always associated with typhoons, which may cross the island almost anywhere.
Appendix III 2 August 2010
The U.S. Army Corps of Engineers (Honolulu) developed the isohyets for the island of Guam (1980). These isohyets represent the local variation in total annual rainfall on Guam with rainfall data before the 1980s. Lately, Guam’s need for an accurate and representative rainfall database, rainfall climatology, and a representative set of rainfall distribution maps, both for monthly and seasonal (rainy season, dry season) periods were addressed by report 2001GU1342B, developed by the U.S. Geological Survey (USGS) and the Water and Environmental Research Institute of the Western Pacific, University of Guam (WERI) in 2001. More recently the USDA National Resources Conservation Service (NRCS, 2008) published the frequency analysis for Guam as Technical Note 3.
METHODOLOGY
There are two predominant ways of gathering data for performing precipitation frequency analysis; namely, annual maximum series and partial duration series. Annual maximum series is performed using highest annual maximum rainfall in a year. Only the greatest rainfall in each year is used. The most common objection to using this series is that it uses only one rainfall event per year. Infrequently, there may be other rainfall events in a year that may out-rank many annual rainfalls. Alternately, the partial duration series selects all rainfall above a certain threshold, without regard to number within any given period. The threshold is a matter of debate with various researchers providing various threshold numbers. Irrespective of the threshold, the most important criteria that needs to be accounted is for is independence of the data. Langbein (1949) suggested that the threshold selected be equal to the lowest annual flood, so that at least one flood in each year is included. If long records are available, he suggested using a threshold so that at least three or four storms a year are included. Wan (2009) suggested that threshold values should be large enough (99th percentile) to support data independence without loss of important information.
Frequency analyses of hydrologic data use probability distributions to relate the magnitude of extreme events to their frequency of occurrence. It is in no way a real representation of actual observed rainfall, but as suggested, is the modeling of extreme events. Historically, lack of short term rainfall data has resulted in researchers using transformation techniques to determine short term rainfall estimates. Empirical relationships have been derived to transform one kind of data into another. Often, annual series are transformed into partial duration series for use in design. Therefore, both annual maximum series and partial duration series are generated for analysis. Langbein (1949) demonstrated that the difference between annual maximum series and partial duration series becomes inconsequential for floods greater than about a 5-year recurrence interval.
This report develops frequency intensity curves for the short duration storms on Guam by the following analyses:
Appendix III 3 August 2010
Annual Maximum Series is developed by the traditional way of selecting the largest storm every year. The results are compared with the series developed by the NRCS.
Partial duration series is developed using the three largest storms in a year. Results are compared to partial duration series developed by NRCS. Whereas NRCS used generalized transformation techniques to develop partial duration series for Guam, this study utilizes the three largest storms selected from each year of record of a rain gauge on Guam.
An Actual Data Series based on selection of the actual storms is developed. This series selects storms that are at least an hour in duration. One such storm, a one-hour duration storm, is selected from each year of record to develop the series. This series will be compared with the partial duration series developed in this report and primarily used to develop a one-hour design storm distribution for use in roadway drainage design.
DATA SELECTION
Though a significant amount of data is available, short-term (such as 15 minute and hourly) rainfall data is available only for limited stations. There are also limitations in the data such as number of days missing; gauge malfunctions, and others. Hydrosphere made an attempt to identify the data with inferior quality with identifiers. Any data with identifiers associated with bad quality were not included in the study, even though they might have been the significant storm events. Short-term rainfall distribution and selection of design storms is the focus of this study. Rainfall gauges have existed on Guam since the 1940s, but useful and long-duration rainfall data is available only for limited stations (NRCS 2008). Therefore, two major assumptions adopted for analyzing rainfall data were:
1. At least 20 years of rainfall data should be available from stations.
2. Since the objective of the study is to determine IDF for roadway-related facility design purposes, only stations that experience maximum rainfall and have 15-minute data available, should be investigated.
A detailed investigation into the rainfall data obtained from Hydrosphere (2009) suggested that 15-minute rainfall data is available only at three precipitation gauging stations -- Fena Lake, Piti, and Inaranjan. The study of the newly-developed annual rainfall distribution map for Guam (USGS, 2001) also suggests that south-central Guam experiences higher total annual rainfall than northern Guam. See Figure 1 for the location of the Fena Lake rainfall gauge on the annual rainfall distribution map developed by USGS. Therefore, the study stations were further limited to the Fena Lake station (See Table 1), which is centrally located in the south half of the island and has a long record consistent with the other two gauges (Piti and Inaranjan).
Appendix III 4 August 2010
Station
Name
Station
Number
Observation
Interval
Years
of
Record
Latitude
Degrees
Minutes
Longitude
Degrees
Minutes
Elevation
Feet
Fena
Lake
914200 15,60 28 13°22’N 144°42’E 60
Table 1: Fena Lake Station
FREQUENCY ANALYSIS
Frequency analysis is performed using HYFRAN software developed by INRS-Eau in collaboration with Hydro-Quebec Hydraulic Services, Canada. HYFRAN fits statistical distributions to random sample data sets. As shown in Table 2, there are 17 distributions available in the software. The Fena Lake data was analyzed for each series and the distribution was selected on the basis of goodness of fit. The software provides goodness of fit measure using Chi-Square, Empirical moment, and Shapiro-Wilk test.
Distribution Estimation Methods*
Exponential MV Fuites Law MV, MM, Mn0/n, MMI Gamma MV, MM Generalized Gamma MV, MM Inverse Gamma MV, MM GEV MV, MM, MMP Gumbel MV, MM, MMP Halphen A, B, et B -1 MV Log-Normal MV Log-Normal 3 param. MV, MM Log-Pearson III SAM, MM (i.e. BOB), WRC Law mixed Log-Normal MV Law mixed Weibull MV Normal MV Generalized Pareto MM, MMP Pearson MV, MM Weibull MV, MM
Table 2: Statistical Distribution in HYFRAN
Appendix III 5 August 2010
Where * MV: maximum likelihood method
* MM: moment method
* MMP: pondered moment method
* Mn0/n : method using n0/n
* MMI : mixed method (Mn0/n and MM)
* SAM: Sundry Averages Method
* WRC: applied moment method on the log of observations.
Figure 1: Annual Rainfall Distribution Map for Guam Showing Location of Fena Lake Rain Gauge Station (Adopted from Report 2001GU1342B)
Appendix III 6 August 2010
DEVELOPMENT OF SHORT-TERM RAINFALL INTENSITIES
Once a distribution is fitted statistically to the data, the standard software output is generated. The output provides rainfall depths for various return periods. This rainfall depth (inches) is converted into rainfall intensity (inches per hour) for the corresponding period by multiplying the corresponding time fraction. For instance, 15 minutes of rainfall depth is multiplied by four to obtain the intensity per hour.
The intensity duration curve is plotted for each return period and a trend line (statistical curve fitting) is performed to develop the intensity duration equation. The equation is then used to extract short-term rainfall intensities.
Annual Maximum Series
A study of annual maximum precipitation frequency for Fena Lake station was conducted to characterize frequency of precipitation. This report presents the frequency of annual maximum precipitation totals for durations of 15 and 30 minutes and 1 hour; and for recurrence intervals of 2, 5, 10, 25, 50 and 100 years. Data collected from Fena station from 1980 through 2008 was used to compile annual maximum precipitation totals for the calendar year, the same period used in NRCS precipitation-frequency studies, except with the additional years since 2005. See Attachment 1 for the data set.
Results
The Lognormal distribution was found to statistically fit the rainfall data for all durations, and the results shown in Table 3 were obtained. See Attachment 1 for the output from HYFRAN software.
Return Period
Years
15 Min. 30 Min. 60 Min.
100 1.57 2.3 3.4 50 1.47 2.16 3.13 25 1.37 2.01 2.86 10 1.22 1.80 2.48 5 1.10 1.63 2.17 2 0.9 1.34 1.68
Table 3: Depth (inches) - Duration Summary - Annual Maximum Series
Rainfall depths were converted to rainfall intensity in inches per hour by multiplying by the time fraction (see Table 4).
Appendix III 7 August 2010
Return Period
Years
15 Min. 30 Min. 60 Min.
100 6.28 4.60 3.40 50 5.88 4.32 3.13 25 5.48 4.02 2.86 10 4.88 3.60 2.48 5 4.40 3.26 2.17 2 3.59 2.68 1.68
Table 4: Intensity (inches/hour) - Duration Summary - Annual Maximum Series
The intensities obtained were plotted using Microsoft Excel. Curves were developed using the trend line function (curve fitting) to obtain the best fit equation for the curve. Table 5 shows the IDF relationships along with the root mean square (R2) value of each equation. R2 value is indicative of the best statistical fit, with values closer to one signifying best fit.
Return Period Years
IDF Equation R2 Value
100 y = 20.79x-0.443 R² = 0.9999 50 y = 20.198x-0.455 R² = 0.9998 25 y = 19.619x-0.469 R² = 0.9993 10 y = 18.519x-0.488 R² = 0.9966 5 y = 17.82x-0.51 R² = 0.9924 2 y = 16.317x-0.548 R² = 0.9828
Table 5: IDF Equations for Intensities - Annual Maximum Series
These equations were used to extrapolate the curve to obtain short-term rainfall intensities. See Table 6.
Time (Min.) 2-Year 5-Year 10-Year 25-Year 50-Year
5 6.75 8.44 9.22 9.71 10.19 10 4.62 6.02 6.66 7.08 7.50 15 3.70 4.94 5.51 5.89 6.26 20 3.16 4.29 4.81 5.17 5.51 25 2.80 3.85 4.34 4.67 5.00 30 2.53 3.52 3.98 4.30 4.61 35 2.33 3.27 3.70 4.01 4.30 40 2.16 3.06 3.48 3.77 4.06 45 2.03 2.89 3.29 3.57 3.85 50 1.91 2.74 3.13 3.41 3.67 55 1.82 2.62 3.00 3.26 3.52 60 1.73 2.51 2.88 3.14 3.39
Table 6: Rainfall Intensity (inches/hour) – Annual Maximum Series
Appendix III 8 August 2010
The resulting short-term IDF curves, as derived from the Annual Maximum Series, are shown in Figure 2.
Figure 2: Rainfall Intensity Curves – Annual Maximum Series
Partial Duration Series
As discussed above, it is important for stormwater management to consider design using multiple storms rather than the highest storm in any given year. As suggested by Langbein (1949), three of the largest storms each year were considered for the development of the partial duration series. This report presents the frequency of partial duration precipitation totals for durations of 15 minutes, 30 minutes, and 1 hour; and for recurrence intervals of 2, 5, 10, 25, 50, and 100 years. Data collected from Fena Lake station from 1980 through 2008 was used to compile annual maximum precipitation totals for the calendar year, the period used in previous precipitation-frequency studies. See Attachment 2 for the sample data set.
Results
The Log Pearson Type 3 curve was found to statistically fit the rainfall data for 15 minutes, whereas Lognormal and Gamma distributions were found statically appropriate for 30 minutes and 60 minutes, respectively. The results are shown in Table 7, which shows the rainfall depth
Appendix III 9 August 2010
for corresponding return periods, as obtained from analysis (see Attachment 2 for output from HYFRAN software).
Return Period
Years
15 Min. 30 Min. 60 Min.
100 1.45 2.09 3.01 50 1.34 1.96 2.79 25 1.23 1.83 2.55 10 1.08 1.64 2.2 5 0.96 1.48 1.91 2 0.77 1.21 1.42
Table 7: Rainfall Depth (inches) - Duration Summary - Partial Duration Series
Rainfall depths were converted to rainfall intensities by multiplying by the time fraction (see Table 8).
Return Period
Years
15 Min. 30 Min. 60 Min.
100 5.8 4.18 3.01 50 5.36 3.92 2.79 25 4.92 3.66 2.55 10 4.32 3.28 2.2 5 3.83 2.96 1.91 2 3.08 2.42 1.42
Table 8: Intensity (inches/hour) - Duration Summary - Partial Duration Series
The intensities obtained were plotted using Microsoft Excel. The data points were trend-lined (curve-fitted) to obtain the best fit equation for the curve. Table 9 shows the Intensity duration relationships, along with the root mean square value of each equation.
Return Period
Years
IDF Equation R2 Value
100 y = 20.891x-0.473 R² = 1
50 y = 19.278x-0.471 R² = 0.9994
25 y = 17.958x-0.474 R² = 0.9967
10 y = 16.479x-0.487 R² = 0.9889
2 y = 14.674x-0.559 R² = 0.9548
Table 9: IDF Equations for Intensities- Partial Duration Series
These equations were used to extrapolate the curve to obtain short-term rainfall intensities (see Table 10).
Appendix III 10 August 2010
Time (Min.) 2-Year 10-Year 25-Year 50-Year
5 6.0 7.5 9.0 9.8
10 4.1 5.4 6.5 7.0
15 3.2 4.4 5.4 5.8
20 2.7 3.8 4.7 5.1
25 2.4 3.4 4.2 4.6
30 2.2 3.1 3.9 4.2
35 2.0 2.9 3.6 3.9
40 1.9 2.7 3.4 3.6
45 1.7 2.6 3.2 3.5
50 1.6 2.45 3.1 3.3
55 1.6 2.34 2.9 3.1
60 1.73 2.88 3.14 3.39
Table 10: Rainfall Intensities (in/hr) – Partial Duration Series
The resulting short-term IDF curve, as derived from the Partial Duration Series, is shown in Figure 3.
Figure 3: Rainfall Intensity Curves – Partial Duration Series
Appendix III 11 August 2010
One – Hour Storm-Based Analysis Series (One-Hour Duration Series)
The above two analyses were performed in accordance with the standard practices followed by the researchers and hydrologists. It is important to note that the procedures described above were developed for performing flood frequency analysis and adopted for precipitation analysis. It is also noteworthy that in the procedure described above, selection of events was carefully performed to ensure that the flood events were independent. For precipitation analysis, this assumption should not be a limiting factor, because the occurrence of short-duration rainfall events is to a large extent independent; that is, any storm in a given day should not affect the next storm either in the same day or any other day. Based on this assumption, a daily peak series was developed for a one-hour storm.
The series included daily peak storm values to mimic the real rainfall scenario. The series of storms selected had a duration of at least one hour or more. One hour or more storms are defined as storms for which the actual duration was at least one hour from the 15-minute rainfall data; that is, 15-minute rainfall data showed at least four consecutive positive values in a day. Also, the most intense 15-minute storms were selected. These were storms demonstrating maximum rainfall depth for the first 15 minutes, even though these storms may not produce the maximum rainfall total in one hour. The reason for this type of selection is to analyze the rainfall distribution of storms that start as intense storms and last at least an hour. This report presents the frequency of actual precipitation totals for durations of 15 minutes, 30 minutes, and 1 hour; and for recurrence intervals of 2, 5, 10, 25, 50, and 100 years. Data collected from the Fena Lake station from 1980 through 2008 was used to compile the annual maximum one-hour storm precipitation totals for the calendar year, the period used in previous precipitation-frequency studies. See Attachment 3 for the data sample.
Results
The Lognormal curve was found to statistically fit all rainfall data for all durations. The following results were obtained. See Attachment 3 for output from HYFRAN software. Table 11 shows the rainfall depth for corresponding return periods, as obtained from analysis.
Return Period
Years
15 Min. 30 Min. 60 Min.
100 1.14 2.07 3.00 50 1.00 1.83 2.71 25 0.87 1.60 2.42 10 0.69 1.30 2.03 5 0.56 1.07 1.72 2 0.38 0.74 1.25
Table 11: Depth (in) - Duration Summary – 1 Hour Storm Based Series
Rainfall depths were converted to rainfall intensities by multiplying by the time fraction (see Table 12).
Appendix III 12 August 2010
Return Period
Years
15 Min. 30 Min. 60 Min.
100 4.56 4.14 3.00 50 3.99 3.66 2.71 25 3.46 3.20 2.42 10 2.78 2.60 2.03 5 2.26 2.14 1.72 2 1.52 1.48 1.25
Table 12: Intensity (in/hr) - Duration Summary – 1-Hour Duration Series
The intensities obtained were plotted using Microsoft Excel. The plotted points were trend-lined (curve-fitted) to obtain the best fit equation for the curve. Table 13 shows the intensity duration relationships as developed, along with the root mean square value of each equation.
Return Period Years
IDF Equation R2 Value
100 y = 5.3568e-0.01x R² = 0.987 50 y = 4.6392e-0.009x R² = 0.986
25 y = 3.9787e-0.008x R² =0.985
10 y = 2.5164e-0.006x R² = 0.978
2 y = 1.6496e-0.004x R² = 0.953
Table 13: IDF Equations for Intensities- 1-Hour Storm Series
These equations were used to extrapolate the curve to obtain one-hour rainfall intensities. See table 14 for results of rainfall intensities.
Time (Min.) 2-Year 10-Year 25-Year 50-Year 100-Year
5 1.62 2.44 3.82 4.44 5.10
10 1.58 2.37 3.67 4.24 4.85
15 1.55 2.30 3.53 4.05 4.61
20 1.52 2.23 3.39 3.87 4.39
25 1.49 2.17 3.26 3.70 4.17
30 1.46 2.10 3.13 3.54 3.97
35 1.43 2.04 3.01 3.39 3.77
40 1.41 1.98 2.89 3.24 3.59
45 1.38 1.92 2.78 3.09 3.42
50 1.35 1.86 2.67 2.96 3.25
Appendix III 13 August 2010
Time (Min.) 2-Year 10-Year 25-Year 50-Year 100-Year
55 1.32 1.81 2.56 2.83 3.09
60 1.30 1.76 2.46 2.70 2.94
Table 14: Rainfall Intensities (in/hr) - 1-Hour Storm Series
Resulting short-term IDF curve as derived from the One-Hour Storm Series is shown in Figure 4.
Figure 4: Rainfall Intensity Curves – 1-Hour Storm Series
Analysis for Selection of Storm for Design of Stormwater Management Facilities
Typically, a 24-hour storm is used as a basis for the design of stormwater facilities. However, many transportation departments use other storms to better fit local conditions. This section describes the analysis of recurrence intervals for various storms as they occur on Guam. An analysis was performed on the actual short-term rainfall (storm) events to ascertain the duration of storms on a daily basis. It was found that between a period of 1980 and 2008, 3,405 storm events were at least 15 minutes long, 1,027 events were at least 30 minutes long, 359 storms lasted 45 minutes or less, and only 185 storms lasted an hour. There was no evidence of a 24-hour duration rainfall event over a period of 28 years. This does not suggest that 24-hour storms
Appendix III 14 August 2010
do not happen, but that the probability is remote. This concurs with the general observation that Guam is frequented by intense short duration storms on an almost daily basis, with a storm rarely lasting more than an hour. A statistical curve fitting was performed on the observations and it was determined that the probability of getting two one-hour storms in a day is less than 2 percent, and the probability of getting three one-hour storms in a day is less than 0.1 percent (see Figure 5). Similarly, the probability of getting two 15-minute storms in a day is 98 percent (see Figure 6).
It is therefore reasonable to design stormwater management facilities in Guam for a one-hour storm as the probability of two one-hour storms happening in a day is less than 2 percent; or in other words, there is a 98 percent chance that two one-hour storms will not occur in one day. Based on this probability, it is safe to infer that probability of back-to-back, one-hour storms is further remote. Considering the probability and economics, it is suggested that stormwater facilities be designed for a one-hour duration storm. For flow control designs, it will be very realistic to detain the one-hour storm and release that runoff volume within a six-hour period to better mimic existing hydrology.
Figure 5: Probability of Recurrence of 1-Hour Storm in a Day
Appendix III 15 August 2010
Figure 6: Probability of Recurrence of 15-Minute Storms in a Day
Comparison of Results and Discussion
The results were compared with results obtained from a recent study by NRCS (2008) (see Table 15 for the annual maximum series and Table 16 for the partial duration series).
Annual Maximum Series
15 Min. 30 Min. 60 Min. Return Period
NRCS Report NRCS Report NRCS Report
2 0.86 0.90 1.33 1.34 1.88 1.68 5 1.04 1.10 1.66 1.63 2.39 2.17
10 1.16 1.22 1.88 1.80 2.72 2.48 25 1.31 1.37 2.16 2.01 3.15 2.86
Appendix III 16 August 2010
15 Min. 30 Min. 60 Min. Return Period
NRCS Report NRCS Report NRCS Report
50 1.42 1.47 2.36 2.16 3.46 3.13 100 1.53 1.57 2.56 2.30 3.77 3.40
Table 15: Comparison with NRCS - Annual Maximum Series
Partial Duration Series
15 Min. 30 Min. 60 Min.
Return Period
NRCS Report NRCS Report NRCS Report
2 0.91 0.77 1.43 1.21 2.03 1.42 5 1.06 0.96 1.69 1.48 2.44 1.91 10 1.17 1.08 1.89 1.64 2.75 2.20 25 1.31 1.23 2.16 1.83 3.16 2.55 50 1.42 1.34 2.36 1.96 3.47 2.79 100 1.53 1.45 2.56 2.09 3.78 3.01
Table 16: Comparison with NRCS-Partial Duration Series
1-Hour Storm Series vs Partial Duration Series (PDS)
15 Min. 30 Min. 60 Min.
Return Period
PDS 1-Hour PDS 1-Hour PDS 1-Hour
2 0.77 0.38 1.21 0.74 1.42 1.25 5 0.96 0.56 1.48 1.07 1.91 1.72
10 1.08 0.69 1.64 1.30 2.20 2.03 25 1.23 0.87 1.83 1.60 2.55 2.42 50 1.34 1.00 1.96 1.83 2.79 2.71
100 1.45 1.14 2.09 2.07 3.01 3.00
Table 17: Comparison of Actual Duration Series and partial Duration Series
Review of the annual maximum series and partial duration series for Fena Lake station suggests that values obtained by this report are within close proximity with the NRCS values for the extreme event modeling. It is important to note that the partial duration series in NRCS was created using transformation techniques, whereas the partial duration series created in this report was developed from taking into account the three largest storms in a year. In both cases extreme events were modeled.
Appendix III 17 August 2010
Whereas, the comparison of the One-Hour Storm Series with the partial duration series suggests there is a significant difference (see Table 17). One of the reasons for the significant difference is that the partial duration series includes short-duration, more intense, thundershowers that occur over small areas in less than one hour in duration. These extreme events were not selected in development of the one-hour duration series, as it has been generally observed that the extreme events are shorter than a one-hour duration on Guam. Therefore, the one-hour storm series does not represent extreme storm events, but does represent the duration and distribution of actual storms that occur on Guam, which are generally an hour or less in duration.
To analyze the rainfall distribution of the partial duration series and the One-Hour series, the ratios of 15-minute to 60-minute and 30-minute to 60-minute rainfalls were calculated based on the assumption that the rainfall is from a one-hour duration storm. Results are displayed in Table 18. The intent is to determine the approximate location of the storm peak.
15 Min./60 Min. 30 Min./60 Min. Return Period
PDS 1-Hour Series
PDS 1-Hour Series
2 0.54 0.30 0.85 0.59 5 0.50 0.33 0.77 0.62 10 0.49 0.34 0.75 0.64 25 0.48 0.36 0.72 0.66 50 0.48 0.37 0.70 0.68 100 0.48 0.38 0.69 0.69
Table 18: Rainfall Depth Ratios
Rainfall depths for the One-Hour storm modeling are approximately 35 percent lower than the partial duration events. The difference is largest for the 15-minute duration with the difference diminishing to less than 10 percent for the 30-minute to 60-minute ratio for all recurrence intervals. Also, if the ratio of 15-minute to 60-minute and 30-minute to 60-minute is evaluated for both the One-Hour storm and the partial duration series, it suggests that for both scenarios, 70 percent of the one-hour rainfall is captured within the first 30 minutes. But for the 15-minute to 60-minute ratio, it suggests that for the PDS, the range of capture is between 48 to 54 percent, whereas it is 30 to 40 percent for the actual series for all recurrence intervals. This shows that extreme events dump about 20 percent more rainfall within the first 15 minutes. Since the data studied is based on 15 minute records, it can be inferred that most one-hour storms peak within 15 minutes. Since no rainfall data is available for a shorter duration, the exact storm peak time cannot be determined. Even so, this confirms the general observation of the very intense short duration storms on Guam, with longer duration storms being sparse and less intense.
Since the main objective of this report is to determine the intensity, duration, and distribution of short-term rainfall to be used for the design of transportation facilities, it is suggested that a synthetic one-hour rainfall be created with 15-minute rainfall depths adopted from partial
Appendix III 18 August 2010
duration series and adopting one-hour storm rainfall depths for longer durations. This is a more balanced approach as it captures the actual rainfall distribution for the one hour duration storm, while allowing for a more conservative design using the higher intensities of the shorter durations. See Table 19 for recommended synthetic rainfall depths by duration and storm frequencies.
Return Period 15 Min. 30 Min. 60 Min.
2 0.77 0.74 1.25 5 0.96 1.07 1.72
10 1.08 1.3 2.03 25 1.23 1.6 2.42 50 1.34 1.83 2.71
100 1.45 2.07 3.0
Table19: Synthetic Rainfall Depth (in)-Duration for the 1-Hour Storm
CONCLUSION
Extreme events modeling revealed similarities between results obtained in this report with other researchers; whereas actual rainfall events were shown to be significantly shorter in duration than the 6-hour to 24-hr events typically used for design of stormwater management facilities.
The recommended storm distribution is a hybrid curve based on 15-minute rainfall from the partial duration series and the milder 30- and 60-minute rainfalls from the One Hour storm series. The subsequent rainfall is conservative for durations shorter than 15 minutes and mimics actual rainfall for longer durations. In support of the synthetic rainfall intensities, Chip Guard (NRCS, 2008) suggests using five-minute rainfall intensities in range of 5.4 to 11.4 inches per hour for a return period of 2 years to 100 years. These results are comparable and within that range.
In order to balance the economics and safety, it is recommended that a one-hour synthetic design rainfall be used for the design of highway stormwater facilities for Guam. Stormwater management systems that will be developed using the selected design storm events and intensity curves will size the drainage facilities to better match actual rainfall conditions. This will help the designer to prepare an efficient design that provides for the public safety, while minimizing project costs.
The following tables and graphs present the recommended rainfall depth duration curves, intensity duration curves, and cumulative rainfall distribution for the one-hour storm.
Appendix III 19 August 2010
Return Period Years
15 Min. 30 Min. 60 Min.
2 3.08 1.48 1.25 5 3.84 2.14 1.72 10 4.32 2.60 2.03 25 4.92 3.20 2.42 50 5.36 3.66 2.71 100 5.80 4.14 3.00
Table 20: Rainfall Intensity (in/hr) - Synthetic 1-Hour Rainfall
Return Period
Years
IDF Equation R2 Value
100 y = 20.971x-0.476 R² = 0.999 50 y = 20.04x-0.492 R² = 0.995
25 y = 19.187x-0.512 R² =0.985
10 y = 18.086x-0.545 R² = 0.962
2 y = 16.322x-0.651 R² = 0.885
Table 21: Rainfall Intensity Equations-Synthetic 1-Hour Rainfall
Time (Min.) 2-Year 10-Year 25-Year 50-Year 100-Year
5 5.72 7.52 8.43 9.08 9.75 10 3.65 5.16 5.91 6.46 7.01 15 2.80 4.13 4.81 5.29 5.78 20 2.32 3.53 4.15 4.59 5.04 25 2.01 3.13 3.70 4.11 4.53 30 1.78 2.83 3.37 3.76 4.15 35 1.61 2.61 3.12 3.49 3.86 40 1.48 2.42 2.91 3.26 3.62 45 1.37 2.27 2.74 3.08 3.43 50 1.28 2.14 2.60 2.92 3.26 55 1.20 2.04 2.47 2.79 3.11 60 1.14 1.94 2.37 2.67 2.99
Table 22: Rainfall Intensities - Synthetic 1-Hour Rainfall
Appendix III 20 August 2010
Figure 7: Intensity Duration Curve - Synthetic 1-Hour Rainfall
Figure 8: Design Rainfall Distribution for 1-Hour Storm
Appendix III 21 August 2010
Time (Min.) Rainfall Fraction
5 0.315 10 0.435 15 0.525 20 0.600 25 0.666 30 0.725 35 0.778 40 0.828 45 0.875 50 0.919 55 0.960 60 1.000
Table 23: One Hour Synthetic Rainfall Fractions
References
Hydrosphere, 2009, Email Correspondence between Parsons Transportation Group and Mr. Ben Harding, Principal Engineer, AMEC Earth and Environmental, Inc (C/o Hydrosphere Inc., CO).
Lander, M. A., Guard, C. P., June 2003, Creation of a 50-Year Rainfall Database, A Rainfall Climatology, and Annual Rainfall Distribution Map of Guam, Water and Environment Research Institute (WERI) of the Western Pacific University of Guam, Technical Report No. 102.
Langbein W.B.; December 1949, Annual and Partial Duration Flood Series, Transactions, American Geophysical Union, Vol 30, Number 6.
NRCS, 2008, Technical Note No.3, Rainfall Frequency and Design Rainfall Distribution for Selected Pacific Islands.
US Army Corps of Engineers, 1980; Guam, Storm Drainage Manual, Guam Comprehensive Study, Water and Related Resources, US Army Corps of Engineers, Honolulu District, HI.
Wan Zin Wan, 2009; A Comparative Study of Extreme Rainfall in Peninsular Malaysia: With Reference to Partial Duration and Annual Extreme Series, Sains malaysiana; 38(5); 751-760.
Appendix III 22 August 2010
ATTACHMENT 1
ANNUAL MAXIMUM SERIES
SAMPLE DATA SET 15 MIN 30 MIN 1 HR
1980 1 1.7 2.6
1981 1 1.5 1.9
1982 1 1 1.2
1983 0.8 1 1.4
1984 1 1.5 2.4
1985 0.9 1.3 1.8
1986 0.9 1.5 1.8
1987 0.6 1.1 1.2
1988 1.7 1.7 1.8
1989 0.8 1.1 1.7
1990 0.6 1 1.2
1991 0.9 1.5 1.7
1992 1.1 1.9 2.1
1993 0.9 1.3 2
1994 1 1.4 1.8
1995 0.9 1.7 2.4
1996 0.9 1.7 2.4
1997 1.1 2 3.5
1998 0.9 1.2 1.3
1999 0.8 1.4 1.5
2000 0.7 1 1.3
2001 0.9 1.4 2.3
2002 0.8 1.5 1.3
2003 0.7 1 1.4
2004 1.2 1.3 1.7
2005 1.5 2 1.9
2006 0.7 1.2 1.3
2007 0.9 0.9 0.9
2008 0.6 1 1.1
Appendix III 23 August 2010
Hyfran Results
Appendix III 24 August 2010
Fena‐ 15 mins Results of the fitting Lognormal (Maximum Likelihood) Number of observations 29 Parameters mu ‐0.10808 sigma 0.240697 Quantiles q = F(X) : non‐exceedance probability T = 1/(1‐q)
T q XT Standard deviation
Confidence interval (95%)
10000 0.9999 2.2 0.281 1.65 2.75
2000 0.9995 1.98 0.228 1.54 2.43
1000 0.999 1.89 0.206 1.49 2.29
200 0.995 1.67 0.157 1.36 1.98
100 0.99 1.57 0.137 1.3 1.84
50 0.98 1.47 0.117 1.24 1.7
25 0.96 1.37 0.0984 1.18 1.56
20 0.95 1.33 0.0924 1.15 1.51
10 0.9 1.22 0.0743 1.08 1.37
5 0.8 1.1 0.0574 0.986 1.21
3 0.6667 0.996 0.0466 0.904 1.09
2 0.5 0.898 0.0401 0.819 0.976
1.4286 0.3 0.791 0.0378 0.717 0.865
1.25 0.2 0.733 0.0383 0.658 0.808
1.1111 0.1 0.659 0.0401 0.581 0.738
1.0526 0.05 0.604 0.0418 0.522 0.686
1.0204 0.02 0.547 0.0437 0.462 0.633
1.0101 0.01 0.513 0.0447 0.425 0.6
1.005 0.005 0.483 0.0455 0.394 0.572
1.001 0.001 0.427 0.0465 0.335 0.518
1.0005 0.0005 0.407 0.0467 0.315 0.498
1.0001 0.0001 0.367 0.0468 0.275 0.458
Appendix III 25 August 2010
Appendix III 26 August 2010
Fena‐ 30 mins Results of the fitting Lognormal (Maximum Likelihood) Number of observations 29 Parameters mu 0.290497 sigma 0.232671 Quantiles q = F(X) : non‐exceedance probability T = 1/(1‐q) T q XT Standard deviation Confidence interval (95%)
10000 0.9999 3.18 0.392 2.41 3.95 2000 0.9995 2.88 0.319 2.25 3.5 1000 0.999 2.74 0.289 2.18 3.31 200 0.995 2.43 0.222 2 2.87 100 0.99 2.3 0.194 1.92 2.68 50 0.98 2.16 0.166 1.83 2.48 25 0.96 2.01 0.14 1.74 2.28 20 0.95 1.96 0.131 1.7 2.22 10 0.9 1.8 0.106 1.59 2.01 5 0.8 1.63 0.0821 1.47 1.79 3 0.6667 1.48 0.0668 1.35 1.61 2 0.5 1.34 0.0578 1.22 1.45
1.4286 0.3 1.18 0.0547 1.08 1.29 1.25 0.2 1.1 0.0555 0.99 1.21
1.1111 0.1 0.992 0.0583 0.878 1.11 1.0526 0.05 0.912 0.0611 0.792 1.03 1.0204 0.02 0.829 0.0639 0.704 0.954 1.0101 0.01 0.778 0.0656 0.65 0.907 1.005 0.005 0.734 0.0668 0.603 0.865 1.001 0.001 0.651 0.0686 0.517 0.786
1.0005 0.0005 0.622 0.0691 0.486 0.757 1.0001 0.0001 0.563 0.0695 0.427 0.699
Appendix III 27 August 2010
Appendix III 28 August 2010
Fena‐ hourly
Results of the fitting
Lognormal (Maximum Likelihood)
Number of observations 29
Parameters
mu 0.517041
sigma 0.304168
Quantiles
q = F(X) : non‐exceedance probability T = 1/(1‐q)
T q XT Standard deviation Confidence interval (95%)
10000 0.9999 5.2 0.839 3.55 6.84
2000 0.9995 4.56 0.663 3.26 5.86
1000 0.999 4.29 0.591 3.13 5.45
200 0.995 3.67 0.437 2.82 4.53
100 0.99 3.4 0.375 2.67 4.14
50 0.98 3.13 0.316 2.51 3.75
25 0.96 2.86 0.26 2.35 3.37
20 0.95 2.77 0.242 2.29 3.24
10 0.9 2.48 0.19 2.1 2.85
5 0.8 2.17 0.143 1.89 2.45
3 0.6667 1.91 0.113 1.69 2.13
2 0.5 1.68 0.0947 1.49 1.86
1.4286 0.3 1.43 0.0863 1.26 1.6
1.25 0.2 1.3 0.0857 1.13 1.47
1.1111 0.1 1.14 0.0873 0.965 1.31
1.0526 0.05 1.02 0.089 0.842 1.19
1.0204 0.02 0.898 0.0905 0.72 1.08
Appendix III 29 August 2010
T q XT Standard deviation Confidence interval (95%)
1.0101 0.01 0.826 0.091 0.648 1
1.005 0.005 0.766 0.0911 0.587 0.945
1.001 0.001 0.655 0.0902 0.478 0.832
1.0005 0.0005 0.616 0.0895 0.441 0.792
1.0001 0.0001 0.541 0.0873 0.37 0.712
Appendix III 30 August 2010
ATTACHMENT 2
PARTIAL DURATION SERIES
SAMPLE DATA SET
15 min high 2nd high 3rd high
1 1980‐01‐01 0.9 1980‐01‐01 0.8 1980‐01‐01
1 1981‐01‐01 0.9 1981‐01‐01 0.7 1981‐01‐01
1 1982‐01‐01 0.9 1982‐01‐01 0.7 1982‐01‐01
0.8 1983‐01‐01 0.7 1983‐01‐01 0.7 1983‐01‐01
1 1984‐01‐01 0.9 1984‐01‐01 0.9 1984‐01‐01
0.9 1985‐01‐01 0.7 1985‐01‐01 0.7 1985‐01‐01
0.9 1986‐01‐01 0.9 1986‐01‐01 0.8 1986‐01‐01
0.6 1987‐01‐01 0.6 1987‐01‐01 0.5 1987‐01‐01
1.7 1988‐01‐01 0.9 1988‐01‐01 0.7 1988‐01‐01
0.8 1989‐01‐01 0.6 1989‐01‐01 0.6 1989‐01‐01
0.6 1990‐01‐01 0.6 1990‐01‐01 0.5 1990‐01‐01
0.9 1991‐01‐01 0.9 1991‐01‐01 0.8 1991‐01‐01
1.1 1992‐01‐01 0.8 1992‐01‐01 0.7 1992‐01‐01
0.9 1993‐01‐01 0.7 1993‐01‐01 0.6 1993‐01‐01
1.00 1994‐01‐01 1.00 1994‐01‐01 1 1994‐01‐01
0.9 1995‐01‐01 0.8 1995‐01‐01 0.8 1995‐01‐01
0.9 1996‐01‐01 0.8 1996‐01‐01 0.8 1996‐01‐01
1.1 1997‐01‐01 0.9 1997‐01‐01 0.9 1997‐01‐01
0.9 1998‐01‐01 0.8 1998‐01‐01 0.7 1998‐01‐01
0.8 1999‐01‐01 0.8 1999‐01‐01 0.7 1999‐01‐01
0.7 2000‐01‐01 0.6 2000‐01‐01 0.6 2000‐01‐01
0.9 2001‐01‐01 0.8 2001‐01‐01 0.7 2001‐01‐01
0.8 2002‐01‐01 0.7 2002‐01‐01 0.7 2002‐01‐01
0.7 2003‐01‐01 0.6 2003‐01‐01 0.6 2003‐01‐01
1.2 2004‐01‐01 0.9 2004‐01‐01 0.7 2004‐01‐01
1.5 2005‐01‐01 1.2 2005‐01‐01 1.1 2005‐01‐01
0.7 2006‐01‐01 0.7 2006‐01‐01 0.6 2006‐01‐01
0.9 2007‐01‐01 0.6 2007‐01‐01 0.2 2007‐01‐01
0.6 2008‐01‐01 0.6 2008‐01‐01 0.4 2008‐01‐01
Appendix III 31 August 2010
30 min high 2nd high 3rd high
1.7 1980‐01‐01 1.3 1980‐01‐01 1.3
1.5 1981‐01‐01 1.2 1981‐01‐01 1.1
1 1982‐01‐01 1 1982‐01‐01 0.9
1 1983‐01‐01 1 1983‐01‐01 1
1.5 1984‐01‐01 1.4 1984‐01‐01 1.2
1.3 1985‐01‐01 1.2 1985‐01‐01 1.2
1.5 1986‐01‐01 1.4 1986‐01‐01 1.3
1.1 1987‐01‐01 1.1 1987‐01‐01 1
1.7 1988‐01‐01 1.7 1988‐01‐01 1.1
1.1 1989‐01‐01 1 1989‐01‐01 0.9
1 1990‐01‐01 0.9 1990‐01‐01 0.9
1.5 1991‐01‐01 1.3 1991‐01‐01 1.3
1.9 1992‐01‐01 1.3 1992‐01‐01 1.2
1.3 1993‐01‐01 1.1 1993‐01‐01 1.1
1.4 1994‐01‐01 1.4 1994‐01‐01 1.4
1.7 1995‐01‐01 1.6 1995‐01‐01 1.6
1.7 1996‐01‐01 1.4 1996‐01‐01 1.3
2 1997‐01‐01 1.8 1997‐01‐01 1.7
1.2 1998‐01‐01 1.1 1998‐01‐01 1
1.4 1999‐01‐01 1.2 1999‐01‐01 1
1 2000‐01‐01 0.9 2000‐01‐01 0.8
1.4 2001‐01‐01 1.4 2001‐01‐01 1.3
1.5 2002‐01‐01 1.3 2002‐01‐01 1
1 2003‐01‐01 1 2003‐01‐01 0.9
1.3 2004‐01‐01 1.2 2004‐01‐01 1.2
2 2005‐01‐01 1.8 2005‐01‐01 1.4
1.2 2006‐01‐01 1.2 2006‐01‐01 1.1
0.9 2007‐01‐01 0.9 2007‐01‐01 0.7
1 2008‐01‐01 0.9 2008‐01‐01 0.6
Appendix III 32 August 2010
hourly high 2nd high 3rd high
2.6 1980‐01‐01 2.5 1980‐01‐01 2.4 1980‐01‐01
1.9 1981‐01‐01 1.6 1981‐01‐01 1.3 1981‐01‐01
1.2 1982‐01‐01 1.1 1982‐01‐01 1 1982‐01‐01
1.4 1983‐01‐01 1.1 1983‐01‐01 1 1983‐01‐01
2.4 1984‐01‐01 2.1 1984‐01‐01 1.6 1984‐01‐01
1.8 1985‐01‐01 1.5 1985‐01‐01 1.1 1985‐01‐01
1.8 1986‐01‐01 1.8 1986‐01‐01 1.6 1986‐01‐01
1.2 1987‐01‐01 1.1 1987‐01‐01 1 1987‐01‐01
1.8 1988‐01‐01 1.7 1988‐01‐01 1.4 1988‐01‐01
1.7 1989‐01‐01 0.9 1989‐01‐01 0.8 1989‐01‐01
1.2 1990‐01‐01 1.1 1990‐01‐01 1.1 1990‐01‐01
1.7 1991‐01‐01 1.5 1991‐01‐01 1.5 1991‐01‐01
2.1 1992‐01‐01 2.1 1992‐01‐01 1.6 1992‐01‐01
2 1993‐01‐01 1.1 1993‐01‐01 1 1993‐01‐01
1.8 1994‐01‐01 1.5 1994‐01‐01 1.2 1994‐01‐01
2.4 1995‐01‐01 2.2 1995‐01‐01 2 1995‐01‐01
2.4 1996‐01‐01 1.7 1996‐01‐01 1.2 1996‐01‐01
3.5 1997‐01‐01 3 1997‐01‐01 2.1 1997‐01‐01
1.3 1998‐01‐01 1.1 1998‐01‐01 0.9 1998‐01‐01
1.5 1999‐01‐01 1.3 1999‐01‐01 1.1 1999‐01‐01
1.3 2000‐01‐01 1.2 2000‐01‐01 1.2 2000‐01‐01
2.3 2001‐01‐01 1.2 2001‐01‐01 1.2 2001‐01‐01
1.3 2002‐01‐01 1.1 2002‐01‐01 1.1 2002‐01‐01
1.4 2003‐01‐01 0.9 2003‐01‐01 0.9 2003‐01‐01
1.7 2004‐01‐01 1.7 2004‐01‐01 1.5 2004‐01‐01
1.9 2005‐01‐01 1.3 2005‐01‐01 1.1 2005‐01‐01
1.3 2006‐01‐01 1.3 2006‐01‐01 1.3 2006‐01‐01
0.9 2007‐01‐01 0.6 2007‐01‐01 0.3 2007‐01‐01
1.1 2008‐01‐01 0.7 2008‐01‐01 0.7 2008‐01‐01
Appendix III 33 August 2010
Appendix III 34 August 2010
Fena‐ 15 mins PDS
Results of the fitting
Log‐Pearson type 3 (Method of moments (BOB), base = 10)
Number of observations 87
Parameters
alpha 3/29/1900
lambda 4/3/1900
m ###########
Quantiles
q = F(X) : non‐exceedance probability T = 1/(1‐q)
T q XT Standard deviation
Confidence interval (95%)
10000 1/0/1900 2.21 0.57 N/D N/D
2000 1/0/1900 1.94 0.38 1.47 2.73
1000 1/0/1900 1.82 0.31 1.40 2.37
200 1/0/1900 1.56 0.18 1.36 2.21
100 1/0/1900 1.45 0.14 1.26 1.85
50 1/0/1900 1.34 0.10 1.21 1.69
25 1/0/1900 1.23 0.07 1.16 1.54
20 1/0/1900 1.19 0.06 1.08 1.33
10 1/0/1900 1.08 0.04 1.00 1.18
5 1/0/1900 0.96 0.03 0.89 1.03
3 1/0/1900 0.86 0.03 0.80 0.919
2 1/0/1900 0.77 0.02 0.71 0.818
1.4286 1/0/1900 0.68 0.02 0.63 0.713
1.25 1/0/1900 0.63 0.02 0.59 0.658
1.1111 1/0/1900 0.57 0.02 0.53 0.607
1.0526 1/0/1900 0.52 0.03 0.47 0.589
1.0204 1/0/1900 0.48 0.04 0.41 0.585
1.0101 1/0/1900 0.45 0.05 N/D N/D
1.005 1/0/1900 0.43 0.06 N/D N/D
1.001 1/0/1900 0.38 0.07 N/D N/D
1.0005 1/0/1900 0.37 0.08 N/D N/D
1.0001 1/0/1900 0.34 0.09 N/D N/D
Appendix III 35 August 2010
Appendix III 36 August 2010
Fena‐ 30 mins PDS Results of the fitting Lognormal (Maximum Likelihood) Number of observations 87 Parameters mu 0.19187 sigma 0.23521 Quantiles q = F(X) : non‐exceedance probability T = 1/(1‐q) T q XT Standard deviation Confidence interval (95%)
10000 0.9999 2.91 0.207 2.5 3.312000 0.9995 2.63 0.169 2.3 2.961000 0.999 2.51 0.153 2.21 2.81200 0.995 2.22 0.117 1.99 2.45100 0.99 2.09 0.102 1.89 2.2950 0.98 1.96 0.0877 1.79 2.1425 0.96 1.83 0.0737 1.68 1.9720 0.95 1.78 0.0692 1.65 1.9210 0.9 1.64 0.0559 1.53 1.755 0.8 1.48 0.0434 1.39 1.563 0.6667 1.34 0.0354 1.27 1.412 0.5 1.21 0.0306 1.15 1.27
1.4286 0.3 1.07 0.0288 1.01 1.131.25 0.2 0.994 0.0292 0.937 1.05
1.1111 0.1 0.896 0.0306 0.836 0.9561.0526 0.05 0.823 0.0319 0.76 0.8851.0204 0.02 0.747 0.0334 0.682 0.8131.0101 0.01 0.701 0.0342 0.634 0.7681.005 0.005 0.661 0.0348 0.593 0.7291.001 0.001 0.586 0.0357 0.516 0.656
1.0005 0.0005 0.559 0.0359 0.488 0.6291.0001 0.0001 0.505 0.036 0.435 0.576
Appendix III 37 August 2010
Appendix III 38 August 2010
Fena‐60 mins PDS
Results of the fitting
Gamma (Maximum Likelihood)
Number of observations 87
Parameters
alpha 5.125316
lambda 7.605498
Quantiles
q = F(X) : non‐exceedance probability
T = 1/(1‐q)
T q XT Standard deviation Confidence interval (95%)
10000 0.9999 4.35 0.326 N/D N/D
2000 0.9995 3.91 0.275 N/D N/D
1000 0.999 3.71 0.252 N/D N/D
200 0.995 3.23 0.2 N/D N/D
100 0.99 3.01 0.177 N/D N/D
50 0.98 2.79 0.154 2.14 3.57
25 0.96 2.55 0.131 2.13 3.24
20 0.95 2.47 0.124 2.06 2.78
10 0.9 2.2 0.101 1.96 2.43
5 0.8 1.91 0.0795 1.77 2.07
3 0.6667 1.66 0.0653 1.55 1.8
2 0.5 1.42 0.056 1.31 1.56
1.4286 0.3 1.16 0.0523 1.05 1.29
1.25 0.2 1.02 0.0523 0.908 1.14
1.1111 0.1 0.849 0.0533 0.73 0.95
1.0526 0.05 0.722 0.0541 0.598 0.814
1.0204 0.02 0.597 0.0542 0.464 0.682
Appendix III 39 August 2010
1.0101 0.01 0.523 0.0538 0.383 0.606
1.005 0.005 0.462 0.0531 0.317 0.544
1.001 0.001 0.354 0.0507 0.202 0.434
1.0005 0.0005 0.317 0.0494 0.165 0.397
1.0001 0.0001 0.249 0.0463 0.1 0.326
Appendix III 40 August 2010
ATTACHMENT 3
ACTUAL 1-HOUR STORM SERIES
SAMPLE DATA SET
15 MINS 30 MINS 45 MINS 60 MINS
0.4 1980‐06‐01 1.3 1980‐06‐01 2.1 1980‐06‐01 2.2 1980‐06‐01
0.4 1981‐07‐20 0.9 1981‐07‐20 1.3 1981‐07‐20 1.4 1981‐07‐20
0.2 1982‐05‐23 0.8 1982‐05‐23 0.9 1982‐05‐23 1.1 1982‐05‐23
0.2 1983‐09‐08 0.6 1983‐09‐08 0.7 1983‐09‐08 0.8 1983‐09‐08
0.4 1984‐12‐04 0.5 1984‐12‐04 0.6 1984‐12‐04 1.2 1984‐12‐04
0.4 1985‐07‐31 0.6 1985‐07‐31 0.7 1985‐07‐31 0.8 1985‐07‐31
0.6 1986‐02‐27 0.7 1986‐02‐27 0.8 1986‐02‐27 1.2 1986‐02‐27
0.4 1987‐11‐22 0.8 1987‐11‐22 1 1987‐11‐22 1.1 1987‐11‐22
0.5 1988‐06‐03 1 1988‐06‐03 1.6 1988‐06‐03 1.8 1988‐06‐03
0.2 1989‐04‐20 0.5 1989‐04‐20 0.6 1989‐04‐20 0.8 1989‐04‐20
0.4 1990‐05‐27 1 1990‐05‐27 1.1 1990‐05‐27 1.4 1990‐05‐27
0.4 1991‐05‐31 1.3 1991‐05‐31 1.7 1991‐05‐31 2 1991‐05‐31
0.3 1992‐08‐27 0.5 1992‐08‐27 0.8 1992‐08‐27 0.9 1992‐08‐27
0.3 1993‐09‐25 0.4 1993‐09‐25 0.9 1993‐09‐25 1.1 1993‐09‐25
0.3 1994‐09‐08 0.5 1994‐09‐08 0.8 1994‐09‐08 1.5 1994‐09‐08
0.5 1995‐08‐20 1.2 1995‐08‐20 1.9 1995‐08‐20 2 1995‐08‐20
0.8 1996‐04‐04 1.7 1996‐04‐04 2.2 1996‐04‐04 2.4 1996‐04‐04
0.6 1997‐08‐01 1 1997‐08‐01 1.4 1997‐08‐01 1.9 1997‐08‐01
0.2 1998‐09‐22 0.5 1998‐09‐22 0.7 1998‐09‐22 1 1998‐09‐22
0.5 1999‐02‐15 0.8 1999‐02‐15 1 1999‐02‐15 1.1 1999‐02‐15
0.5 2000‐09‐07 0.8 2000‐09‐07 1.1 2000‐09‐07 1.2 2000‐09‐07
0.6 2001‐07‐13 1.2 2001‐07‐13 1.9 2001‐07‐13 2 2001‐07‐13
0.7 2002‐09‐26 1.3 2002‐09‐26 1.7 2002‐09‐26 1.8 2002‐09‐26
0.3 2003‐07‐16 0.6 2003‐07‐16 0.9 2003‐07‐16 1 2003‐07‐16
0.4 2004‐10‐13 0.5 2004‐10‐13 0.8 2004‐10‐13 1.1 2004‐10‐13
0.2 2005‐08‐26 0.6 2005‐08‐26 1 2005‐08‐26 1.1 2005‐08‐26
0.7 2006‐10‐08 1.1 2006‐10‐08 1.3 2006‐10‐08 1.4 2006‐10‐08
0.1 2007‐05‐09 0.3 2007‐05‐09 0.4 2007‐05‐09 0.5 2007‐05‐09
0.2 2008‐01‐10 0.6 2008‐01‐10 1.2 2008‐01‐10 1.5 2008‐01‐10
Appendix III 41 August 2010
Hyfran Results
Appendix III 42 August 2010
15 MINS FENA STORM BASED ANALYSIS
Results of the fitting
Lognormal (Maximum Likelihood)
Number of observations 29
Parameters
mu ‐1.01197
sigma 0.488046
Quantiles
q = F(X) : non‐exceedance probability
T = 1/(1‐q)
T q XT Standard deviation Confidence interval (95%)
10000 0.9999 2.23 0.578 1.1 3.37
2000 0.9995 1.81 0.422 0.984 2.64
1000 0.999 1.64 0.363 0.931 2.35
200 0.995 1.28 0.244 0.8 1.76
100 0.99 1.13 0.2 0.739 1.52
50 0.98 0.991 0.16 0.677 1.3
25 0.96 0.854 0.125 0.61 1.1
20 0.95 0.811 0.114 0.588 1.03
10 0.9 0.679 0.0838 0.515 0.844
5 0.8 0.548 0.0581 0.434 0.662
3 0.6667 0.448 0.0425 0.365 0.532
2 0.5 0.364 0.0329 0.299 0.428
1.4286 0.3 0.281 0.0273 0.228 0.335
1.25 0.2 0.241 0.0255 0.191 0.291
1.1111 0.1 0.194 0.024 0.147 0.241
1.0526 0.05 0.163 0.0229 0.118 0.208
1.0204 0.02 0.133 0.0216 0.0911 0.176
1.0101 0.01 0.117 0.0206 0.0763 0.157
1.005 0.005 0.103 0.0197 0.0647 0.142
1.001 0.001 0.0804 0.0178 0.0456 0.115
1.0005 0.0005 0.0729 0.017 0.0396 0.106
1.0001 0.0001 0.0592 0.0153 0.0291 0.0892
Appendix III 43 August 2010
Appendix III 44 August 2010
30 MINS FENA STORM BASED ANALYSIS
Results of the fitting
Lognormal (Maximum Likelihood)
Number of observations 29
Parameters
mu ‐0.290007
sigma 0.419978
Quantiles
q = F(X) : non‐exceedance probability
T = 1/(1‐q)
T q XT Standard deviation Confidence interval (95%)
10000 0.9999 3.57 0.795 2.01 5.13
2000 0.9995 2.98 0.597 1.81 4.15
1000 0.999 2.74 0.521 1.72 3.76
200 0.995 2.21 0.363 1.5 2.92
100 0.99 1.99 0.302 1.4 2.58
50 0.98 1.77 0.247 1.29 2.26
25 0.96 1.56 0.196 1.18 1.95
20 0.95 1.49 0.18 1.14 1.85
10 0.9 1.28 0.136 1.02 1.55
5 0.8 1.07 0.0971 0.875 1.26
3 0.6667 0.896 0.0732 0.753 1.04
2 0.5 0.748 0.0584 0.634 0.863
1.4286 0.3 0.6 0.05 0.502 0.699
1.25 0.2 0.526 0.0479 0.432 0.619
1.1111 0.1 0.437 0.0463 0.346 0.528
1.0526 0.05 0.375 0.0453 0.286 0.464
1.0204 0.02 0.316 0.044 0.23 0.402
1.0101 0.01 0.282 0.0428 0.198 0.366
1.005 0.005 0.254 0.0417 0.172 0.335
1.001 0.001 0.204 0.0389 0.128 0.281
1.0005 0.0005 0.188 0.0377 0.114 0.262
1.0001 0.0001 0.157 0.035 0.0884 0.225
Appendix III 45 August 2010
Appendix III 46 August 2010
60 MINS FENA STORM BASED ANALYSIS Results of the fitting Lognormal (Maximum Likelihood) Number of observations 29 Parameters mu 0.257495 sigma 0.365753 Quantiles q = F(X) : non‐exceedance probability T = 1/(1‐q) T q XT Standard deviation Confidence interval (95%)
10000 0.9999 5.04 0.978 3.12 6.962000 0.9995 4.31 0.753 2.84 5.791000 0.999 4.01 0.664 2.71 5.31200 0.995 3.32 0.475 2.39 4.25100 0.99 3.03 0.401 2.24 3.8250 0.98 2.74 0.332 2.09 3.3925 0.96 2.45 0.268 1.93 2.9820 0.95 2.36 0.249 1.87 2.8510 0.9 2.07 0.191 1.69 2.445 0.8 1.76 0.14 1.49 2.033 0.6667 1.51 0.108 1.3 1.732 0.5 1.29 0.0879 1.12 1.47
1.4286 0.3 1.07 0.0775 0.916 1.221.25 0.2 0.951 0.0755 0.803 1.1
1.1111 0.1 0.81 0.0748 0.663 0.9561.0526 0.05 0.709 0.0746 0.563 0.8551.0204 0.02 0.61 0.074 0.465 0.7551.0101 0.01 0.552 0.0732 0.409 0.6961.005 0.005 0.504 0.0721 0.363 0.6461.001 0.001 0.418 0.0692 0.282 0.553
1.0005 0.0005 0.388 0.0678 0.255 0.5211.0001 0.0001 0.332 0.0644 0.206 0.458